The Wilkinson Microwave Anisotropy Probe (WMAP) by reading the cosmic microwave background( CMB ) determined that only 4.6% of the total mass-energy constituent of the universe is made of ordinary atoms or baryons( made of neutrons, protons and electrons).
From the very moment of the big bang till today, some of the visible stuffs that our universe has manifested like dust, gas, planets, stars, galaxies, you and me only accounts for 4.6% of universe. The 95% of the energy density in the universe is in a form that has never been directly detected in the laboratory or was unknown.
What about the rest of the universe ?
23% of the universe contains 'Dark Matter'.
Dark matter is likely to be composed of one or more species of sub-atomic particles that interacts weakly with ordinary matter. Dark matter interacts with ordinary matter through gravity and weak force which are the weakest among the four fundamental forces.
Dark matter neither absorbs or emits light or any other electromagnetic radiation at a significant level so it's invisible. Hence the name 'dark matter'. Dark matter are three types but, when we are talking about dark matter that truly counts then it means 'cold dark matter'.
Fritz Zwicky, an astronomer at the California Institute of Technology, stumbled across the unexplained gravitational effects of in the early 1930s while studying how galaxies move within the Coma Cluster. The gravitational effect can only be explained if there is sufficient mass present within the cluster as gravity is a function of mass( more the mass more the gravity ). But it's not visible. So he named it 'dark matter'.
( measuring orbital speed of stars in galaxies )
In 1970s, astronomers Kent Ford and Vera Cooper Rubin at the Carnegie Institution of Washington began a detailed study about the motion of stars( about 90 stars ) in the nearby galaxy of Andromeda for 2 years.
In a spiral galaxy like Andromeda which contains a central supermassive black hole, other stars and solar systems move around it. So as you move from the center to the edge of the galaxy, the gravitational force gradually decreases. As the gravitational force decreases the orbital speeds of the stars should also decrease. By measuring 'redshift' Ford and Rubin could measure the actual orbital speed of the stars, the same method used by Zwicky himself.
when she analysed the data she found that through out the galaxy the orbital velocities of stars are almost constant. Which is not possible because if the stars towards the edges would move with such massive speeds then they would have been thrown out into the space because the observed gravity is not enough to hold them of in the orbit. So there must be extra mass present in form of huge amounts of unseen matter. For conformation she studied some sixty spiral galaxies over few years and found the same thing.
Her calculations showed that galaxies must contain about ten times as much “dark matter”, that can be accounted for the high orbital speeds of visible stars. In short, at least ninety percent of the mass in galaxies is invisible. This was the 1st prof that Fritz Zwicky was right. In fact dark matter could be a key piece of puzzle to our existence.
( Through the effects of gravitational lensing )
It was not until 1979 that this effect of gravitational lensing was confirmed by observation of the so-called "Twin QSO" SBS 0957+561( the twin quasar ). Later it was found that through weak gravitational lensing, the presence of dark matter can be observed.
Dr. Richard Massey study about the gravitational lensing produced by galaxies or clusters of galaxies. But the gravitational lensing produced by the galaxies in his study was too much bending of light accordance to the amount of matter present. His calculated that there is 5 times more dark matter present in the galaxies than the normal matter which is the same observation as Ford and Rubin found out.
In 2007 Hubble telescope makes the 1st 3D map of the dark matter for a part of the universe and Richard is one of the leads to this group.
For more info, please visit - news.bbc.co.uk
But in the year 2004 two galaxy clusters collided about 3 billion light years away. As the two clusters bumped into with each other at 3 thousand miles per second, the luminous matter in each violently collided with the other and slowed down producing x-rays. But the dark matter did not interact at all, passing right through without disruption. This caused the dark matter to sail ahead, separating each cluster into two components: dark matter in the lead and luminous matter lagging behind.
The shape of the hot gas produced by the collision of the clusters shows significance resemblance with bullet shape, hence the name 'bullet' cluster. One of the famous images in the history.
For more info, please visit - physicsworld.com
2. Through computer simulations
In 1973, Princeton University astronomers Jeremiah Ostriker and James Peebles used numerical simulation to study how galaxies evolve. Applying a technique called N-body simulation, they programmed 300 mass points into their computer to represent groups of stars in a galaxy rotating about a central point ( like in milky-way galaxy ). Their simulated galaxy had more mass points, or stars, toward the center and fewer toward the edge. The simulation started by computing the gravitational force between each pair of mass points from Newton's law and working out how the mass points would move in a small interval of time. By repeating this calculation many times, Ostriker and Peebles were able to track the motion of all the mass points in the galaxy over a long period of time.
Ostriker and Peebles found that gradually, most of the mass points would collapse to a bar-shaped, dense concentration close to the center of the galaxy with only a few mass points at larger radii. This looked nothing like the elegant spiral or elliptical shapes we are used to seeing. However, if they added a static, uniform distribution of mass 3 to 10 times the size of the total mass of the mass points, they found a more recognizable structure would emerge. Ostriker and Peebles had solid numerical evidence that dark matter was necessary to form the types of galaxies we observe in our universe.
But later on computers became more and more powerful. So it was possible to simulate complex shapes like galaxies and galaxy clusters.
Prof Carlos Frenk uses a supercomputer which consists of 1300 computers all working together, a super cosmology machine. He 1st tried to simulate a galaxy from gravity and the matter we see. As it was believed that gravity gradually drew the gases together and where the gas was densely packed, the galaxies were formed in those areas we see today.
Dr. Carlos programmed the necessary stuffs and ran the simulation and waited for the machine to create a galaxy but, what really happened was few stars( both mid sized stars and supermassive stars ) were formed and the supermassive stars exploded( as big star shorter life spans )and the massive energy from the explosions spreads out the gas all over the space, greatly reducing the gas density required for star formation. So it's clear that there was not enough gravity to hold the structure together.
Then Dr. Carlos started adding extra gravity in form of dark matter. Firsly a little amount and gradually five times more than the visible normal matter. Then the simulation slowly started forming the shape of the universe we observe today.
This not only confirms the presence of dark matter but, it explains that dark matter plays an important role in galaxy formation. Even the galaxy clusters and galaxy super clusters are formed because of the dark matter.So dark matter is the glue that sticks everything together in our universe.
What is dark matter made of ?
The most plausible candidate for dark matter is weakly Interacting Massive Particles( WIMPs ), emerge from theory of "Supersymmetry'. Supersymmetry is a theoretical notion by which known elementary particles have supersymmetric partner particles. This theory predicts candidates for dark matter comes under the name, Lightest Supersymmetric Particle (LSP). The LSP could be the Nutralinos, the gravitino or the lightest sneutrino.
WIMPs are made of non-baryonic matter that is they do not have sub atomic particles like protons or neutrons in them. As they are different from the normal matter, they do not interact with they normal matter. Billions of WIMPs are passing right through us every second and the chances of a WIMP interacting with a atom in our body is rare cause sometimes when they come close enough to a nucleus they interact via weak force. But even though the probability is less than 1% or may be much more less, Physicists are counting on it.
WIMPs can be detected in few ways.
1.Accelerator searches
Through experiments in particle accelerators like Large Hydron Collider( LHC ) at CERN, Geneva. As WIMPs are so much non-interactive so if a WIMP is produced, it will simply fly out the accelerator space and hence it's signature will be the missing energy. But still today there is no full-proof evidence of a WIMP detection. Though the window for the existence of the lightest supersymmetry particle 'Nutralinos' is becoming less narrow day by day but, Physicists still have a hope to find it one day.
2.Direct detection WIMPs
A satisfying solution to the dark matter problem would be the detection of WIMPs as they move past and through the Earth. This would also allow measurement of the local density of dark matter and establish beyond doubt that the dark matter is non-baryonic. There are several ways to do this, and currently two methods are being aggressively pursued.
WIMPs rarely interacts with matter but, when it does, it scatter off a nucleus and the nucleus recoils, causing dislocation in the crystal structure, vibrations of the crystal lattice (i.e. produces photons or heat) and also ionization. These signals can be detected. For example, some experiments use kilogram size germanium and silicon crystals( are very stable and still crystal structure at very low tempretures ) and attempt to detect the ionization and photon signals.
Cryogenic Dark Matter Search (CDMS) detector at the Soudan Mine, Minesotta uses this technique in a lab about half a mile underground in a abandoned iron mine. In February 2010, researchers at the Soudan Mine CDMS II experiment announced that they had observed two events that may have been caused by WIMP-nucleus collisions but, not for sure.
Scientists from the XENON collaboration announced a new result from their search for dark matter. The analysis of data taken with the XENON100 detector during 13 months of operation at the Gran Sasso Laboratory (Italy) provided no evidence for the existence of Weakly Interacting Massive Particles (WIMPs). ( content released in 18th July, 2012 )
For more info, please visit - www.interactions.org
Other groups use sodium iodide crystals and look for the scintillation light caused by the ionized electrons or search for crystal dislocations in samples of billion year old mica. Another possibility is to record the recoil of an ionized atom in a gas using drift chamber techniques.
Now particle physicists are proposing a new way to detect dark matter using the molecule of life - www.sciencenews.org
3.Indirect detection of WIMPs
The idea is that if the halo is made of WIMPs, then these WIMPs will have been passing through the Earth and Sun for several billion years. Since WIMPs will occasionally elastically scatter off nuclei in the Sun or Earth, they will occasionally lose enough energy, or change their direction of motion enough, to become gravitationally captured by the Sun or Earth. The orbits of such captured WIMPs will repeatedly intersect the Sun (or Earth) resulting in the eventual settling of the WIMPs into the core. As the number density increases over time, the self annihilation rate will increase. Since ordinary neutrinos can result from WIMP self-annihilation, one predicts a stream of neutrinos coming from the core of the Sun or Earth. Neutrinos easily escape the Solar core. The detectors on Earth capable of detecting neutrinos coming from Sun or Earth have operated for some time. The energy of such neutrinos is roughly 1/2 to 1/3 of the WIMP mass, so these neutrinos are much higher energy than the MeV scale solar neutrinos from nuclear reactions that have already been detected. The higher energy of these WIMP annihilation neutrinos makes them easier to detect than ordinary solar neutrinos and somewhat compensates for their much fewer numbers. Thus the presence of a source of high-energy neutrinos emanating from the centers of the Sun and Earth would be taken as evidence for WIMP dark matter.
For more info, please visit - http://www.snowmass2013.org.
Another candidate is axions. There are other possible dark matter candidates which do not fit into the WIMP framework. The axions arise from attempts to explain why the strong interaction seems to obey a certain symmetry called "CP symmetry". Among other things, CP symmetry would prevent the neutron from having a large electric dipole moment - without it, it's very hard to understand why such a dipole moment has not yet been detected. The best explanation for this, is called "Peccei-Quinn symmetry", and predicts a new light neutral particle called the axion.
For more info, please visit - physicsworld.com
Some astrophysicists are exploring the possibility of dark matter could lie inside the extra dimensions of the 'string theory'. The string theory adds six new dimensions to the four usual ones (three dimensions of space and one dimension for time) and would place the dark matter in these new dimensions which are inaccessible for us; explaining why it would not be detectable. The electromagnetic and nuclear forces (both strong and weak) would be confined in our four dimensions and could not leave them. If that's the case, there could be accompanying particles called Kaluza–Klein particles that account for dark matter. However, these would be even harder to detect.
For more info, please visit - alextorex.wordpress.com
Finally Massive astrophysical Compact Halo Objects( MACHOs )are non-luminous objects that make up the halos around galaxies. MACHOs are thought to be brown dwarf stars, neutron stars, white dwarfs, black holes. Unlike others, MACHOs are made of baryonic matter but, they don,t emit any light of their own so they are hard to detect. MACHOs can be found with gravitational lensing or gravitational microlensing. But there’s not enough ordinary matter (protons, neutrons, electrons) to account for the measured amount of dark matter so MACHOs are somewhat ruled out as a candidate. As we already know they are focusing more on WIMPs.
72% of the universe contains 'Dark energy'
Dark energy is trying to rip our universe apart.
In the early 1920s, astronomer Edwin Hubble, working at the Mount Wilson Observatory in California, discovered that many of the diffuse objects in the sky called nebulae were actually in fact galaxies spreading through out the universe. Later in the decade, Hubble and his assistant Milton L. Humason estimated the distances to galaxies using a technique invented by Henrietta
Swan. This they combined with measurements of the 'red-shifts' of the spectral lines from stars in the galaxies.
In 1929, Hubble and Humason found that the galaxies were generally red-shifted and so were on average receding from us. They found that the amount of red-shift from a galaxy was roughly proportional to its distance from us, that is the galaxies more farther away were moving away with more speed and the galaxies near to us were moving relatively less slowly. So our universe is expanding (This discovery also gave rise to the 'big bang').
The mathematical equation for Hubble's law is as follows:
The 1st candidate should be the 'cosmological constant'( otherwise known as 'vacuum energy' ) from General relativity its self. When Einstein solved his equation of general relativity for the 1st time, he found out that the universe was not static, in fact it was either expanding or contracting. He didn't liked the idea of a expanding universe so Einstein himself added a cosmological constant to his equations of general relativity to counteract the effect of gravity, that is, a constant which counteracts this effect. Friedmann, a Russian mathematician, realized that this was an unstable fix, like balancing a pencil on its point, and proposed an expanding universe model, now called the 'Big Bang theory'. Hubble's study of nearby galaxies showed that actually the universe was in fact expanding.
Einstein regretted, viewed the cosmological constant term as his "greatest mistake".
In 1980 it was put forward as a candidate for the repulsive force that produced the enormous exponential expansion, called 'inflation' of the universe.
Today it is proposed as a candidate for dark energy itself, which is responsible for driving the acceleration of the universe. The cosmological constant is the property of empty space itself.
If you remove all the particles and radiations of a small part of space, then you could call it an 'empty space' but, that does not mean that empty space is 'nothing'. It's not true. In physics because of the 'quantum mechanics' and 'special relativity' in very very small scales, empty space is a bubbling, boiling soup of 'virtual particles' that pops in and out of existence in a time scale so short that you can't see them. These "virtual pairs" consist of one particle with a negative charge and one with a positive charge. They exist for only a tiny fraction of a second before they collide and annihilate each other in a tiny burst of energy. These energy bursts creates ripples of energy in space-time fabric which may be driving the galaxies apart. So empty space has it's energy after all. So 'nothing' is actually something.
In the early universe, everything was closely packed so there was less 'space' fabric existed. The gravity was dominant. The cosmological constant is a property of empty space itself as universe grew over time, the gravitational pull weakened between structures and the cosmological constant dominated the growth of the universe.
For more info, please visit - www.csicop.org
Another candidate for dark energy is is a scalar field called 'quintessence'. It is also proposed as a fundamental 'fifth force', a cosmic chameleon. It is time dependent that is it grows with time ans it is dynamic. So it's some what popular over the cosmological constant( doesn't change with time ).
For more info, please visit - arxiv.org
Another candidate is gravity itself. Einstein's general relativity predicts gravity as warp in the fabric of space-time and we take it for granted. But relativity only holds good for the universe on large scales, on the smallest scales, quantum gravity takes over. Relativity also breaks down in the presence of the strongest gravitational fields, like those at the center of a black hole, called the singularity.
And now we know that something called the 'dark matter', which constitutes 23% of the universe has much strong gravitational effect than the normal matter. The existence of dark matter is one of the key points in the evolution of our universe. Without it everything would have fallen apart.
And now we are becoming aware of something called the 'dark energy',which constitutes 72% of the universe and is accelerating the expansion of the universe. It is producing some 'anti-gravitational' effects.
So maybe there is more to the concept of 'gravity'.
For more info, please visit - news.nationalgeographic.co.in
If you didn't find the above link useful then please try - arxiv.org
So was dark energy always around from the beginning? Did the expansion of our universe ever slowed down? what is going to happen if the rate of expansion of the universe keeps increasing?
The Baryon Oscillation Spectroscopic Survey (BOSS) - part of the Sloan Digital Sky Survey (SDSS-III), relying on data from distant quasars have enabled physicists to produce a detailed 3D "map" of the early universe a whopping 11.5 billion years ago.
Boss measures the Baryonic Acoustic Oscillation (BAO). BAO is the result of the pressure waves or ripples propagating through the universe from it's most earliest phases( from 30,000 years after the big bang ) when everything was a very hot and dense soup of baryons and photons.
BOSS measurements indicate that dark energy was not always the superior force. It indicates that before a critical point 11 billion years ago, the gravitational force was dominant and the universe was slowing down.
If the acceleration continues then the fabric of space-time will be stretched to such an extent that it will be torn apart and the matters will be disintegrated. It is called the 'Big Rip'.
So dark energy remains as the biggest mystery out there today. To hunt for dark energy, a 23-institution consortium called The Dark Energy Survey has built a 570 mega pixel camera called 'the dark energy camera', which will be used it to study four phenomena. Events like
- type 1a supernova
- Baryon acoustic oscillation (BAO)
- Gravitational lensing
- Growth of galaxy clusters over time
For more info, please visit - www.theverge.com
To solve the mystery of the dark matter and dark energy, mathematicians are also stepping forward. Two mathematicians offer a unified theory of dark matter and dark energy by altering Einstein's field equations. They suggested that the laws of conservation of energy and momentum in space-time is valid only when normal matter, dark matter and dark energy are all considered. For normal matter only, the laws will not hold good.
For more info, please visit - newsinfo.iu.edu
The day when the greatest minds will be able to understand what dark matter and dark energy exactly are. That will be the greatest victory for man-kind.
From the very moment of the big bang till today, some of the visible stuffs that our universe has manifested like dust, gas, planets, stars, galaxies, you and me only accounts for 4.6% of universe. The 95% of the energy density in the universe is in a form that has never been directly detected in the laboratory or was unknown.
What about the rest of the universe ?
23% of the universe contains 'Dark Matter'.
Dark matter is likely to be composed of one or more species of sub-atomic particles that interacts weakly with ordinary matter. Dark matter interacts with ordinary matter through gravity and weak force which are the weakest among the four fundamental forces.
Dark matter neither absorbs or emits light or any other electromagnetic radiation at a significant level so it's invisible. Hence the name 'dark matter'. Dark matter are three types but, when we are talking about dark matter that truly counts then it means 'cold dark matter'.
So how was it discovered?
Fritz Zwicky, an astronomer at the California Institute of Technology, stumbled across the unexplained gravitational effects of in the early 1930s while studying how galaxies move within the Coma Cluster. The gravitational effect can only be explained if there is sufficient mass present within the cluster as gravity is a function of mass( more the mass more the gravity ). But it's not visible. So he named it 'dark matter'.
Proofs that dark matter exists
1.From cosmological observations( measuring orbital speed of stars in galaxies )
 |
| Vera Rubin Cooper |
 |
| Kent Ford |
In a spiral galaxy like Andromeda which contains a central supermassive black hole, other stars and solar systems move around it. So as you move from the center to the edge of the galaxy, the gravitational force gradually decreases. As the gravitational force decreases the orbital speeds of the stars should also decrease. By measuring 'redshift' Ford and Rubin could measure the actual orbital speed of the stars, the same method used by Zwicky himself.
when she analysed the data she found that through out the galaxy the orbital velocities of stars are almost constant. Which is not possible because if the stars towards the edges would move with such massive speeds then they would have been thrown out into the space because the observed gravity is not enough to hold them of in the orbit. So there must be extra mass present in form of huge amounts of unseen matter. For conformation she studied some sixty spiral galaxies over few years and found the same thing.
Her calculations showed that galaxies must contain about ten times as much “dark matter”, that can be accounted for the high orbital speeds of visible stars. In short, at least ninety percent of the mass in galaxies is invisible. This was the 1st prof that Fritz Zwicky was right. In fact dark matter could be a key piece of puzzle to our existence.
( Through the effects of gravitational lensing )
It was not until 1979 that this effect of gravitational lensing was confirmed by observation of the so-called "Twin QSO" SBS 0957+561( the twin quasar ). Later it was found that through weak gravitational lensing, the presence of dark matter can be observed.
Dr. Richard Massey study about the gravitational lensing produced by galaxies or clusters of galaxies. But the gravitational lensing produced by the galaxies in his study was too much bending of light accordance to the amount of matter present. His calculated that there is 5 times more dark matter present in the galaxies than the normal matter which is the same observation as Ford and Rubin found out.
In 2007 Hubble telescope makes the 1st 3D map of the dark matter for a part of the universe and Richard is one of the leads to this group.
For more info, please visit - news.bbc.co.uk
But in the year 2004 two galaxy clusters collided about 3 billion light years away. As the two clusters bumped into with each other at 3 thousand miles per second, the luminous matter in each violently collided with the other and slowed down producing x-rays. But the dark matter did not interact at all, passing right through without disruption. This caused the dark matter to sail ahead, separating each cluster into two components: dark matter in the lead and luminous matter lagging behind.
The shape of the hot gas produced by the collision of the clusters shows significance resemblance with bullet shape, hence the name 'bullet' cluster. One of the famous images in the history.
 |
| The bullet cluster |
2. Through computer simulations
 |
| Jeremiah Ostriker |
 |
| James Peebles |
Ostriker and Peebles found that gradually, most of the mass points would collapse to a bar-shaped, dense concentration close to the center of the galaxy with only a few mass points at larger radii. This looked nothing like the elegant spiral or elliptical shapes we are used to seeing. However, if they added a static, uniform distribution of mass 3 to 10 times the size of the total mass of the mass points, they found a more recognizable structure would emerge. Ostriker and Peebles had solid numerical evidence that dark matter was necessary to form the types of galaxies we observe in our universe.
 |
| Carlos Frenk |
Prof Carlos Frenk uses a supercomputer which consists of 1300 computers all working together, a super cosmology machine. He 1st tried to simulate a galaxy from gravity and the matter we see. As it was believed that gravity gradually drew the gases together and where the gas was densely packed, the galaxies were formed in those areas we see today.
Dr. Carlos programmed the necessary stuffs and ran the simulation and waited for the machine to create a galaxy but, what really happened was few stars( both mid sized stars and supermassive stars ) were formed and the supermassive stars exploded( as big star shorter life spans )and the massive energy from the explosions spreads out the gas all over the space, greatly reducing the gas density required for star formation. So it's clear that there was not enough gravity to hold the structure together.
Then Dr. Carlos started adding extra gravity in form of dark matter. Firsly a little amount and gradually five times more than the visible normal matter. Then the simulation slowly started forming the shape of the universe we observe today.
This not only confirms the presence of dark matter but, it explains that dark matter plays an important role in galaxy formation. Even the galaxy clusters and galaxy super clusters are formed because of the dark matter.So dark matter is the glue that sticks everything together in our universe.
What is dark matter made of ?
The most plausible candidate for dark matter is weakly Interacting Massive Particles( WIMPs ), emerge from theory of "Supersymmetry'. Supersymmetry is a theoretical notion by which known elementary particles have supersymmetric partner particles. This theory predicts candidates for dark matter comes under the name, Lightest Supersymmetric Particle (LSP). The LSP could be the Nutralinos, the gravitino or the lightest sneutrino.
WIMPs are made of non-baryonic matter that is they do not have sub atomic particles like protons or neutrons in them. As they are different from the normal matter, they do not interact with they normal matter. Billions of WIMPs are passing right through us every second and the chances of a WIMP interacting with a atom in our body is rare cause sometimes when they come close enough to a nucleus they interact via weak force. But even though the probability is less than 1% or may be much more less, Physicists are counting on it.
WIMPs can be detected in few ways.
1.Accelerator searches
 |
| Large Hydron Collider, CERN |
Through experiments in particle accelerators like Large Hydron Collider( LHC ) at CERN, Geneva. As WIMPs are so much non-interactive so if a WIMP is produced, it will simply fly out the accelerator space and hence it's signature will be the missing energy. But still today there is no full-proof evidence of a WIMP detection. Though the window for the existence of the lightest supersymmetry particle 'Nutralinos' is becoming less narrow day by day but, Physicists still have a hope to find it one day.
2.Direct detection WIMPs
A satisfying solution to the dark matter problem would be the detection of WIMPs as they move past and through the Earth. This would also allow measurement of the local density of dark matter and establish beyond doubt that the dark matter is non-baryonic. There are several ways to do this, and currently two methods are being aggressively pursued.
WIMPs rarely interacts with matter but, when it does, it scatter off a nucleus and the nucleus recoils, causing dislocation in the crystal structure, vibrations of the crystal lattice (i.e. produces photons or heat) and also ionization. These signals can be detected. For example, some experiments use kilogram size germanium and silicon crystals( are very stable and still crystal structure at very low tempretures ) and attempt to detect the ionization and photon signals.
 |
| Graphical representation of what happens when a WIMP interacts with a super cooled Germanium crystal. |
Cryogenic Dark Matter Search (CDMS) detector at the Soudan Mine, Minesotta uses this technique in a lab about half a mile underground in a abandoned iron mine. In February 2010, researchers at the Soudan Mine CDMS II experiment announced that they had observed two events that may have been caused by WIMP-nucleus collisions but, not for sure.
 |
| Dan Bauer, CDMS project manager and Fermilab scientist, removes one tower of detectors used in the Cryogenic Dark Matter Search experiment. Credit: Fermilab. |
 |
| Detectors made of germanium and silicon |
 |
| XENON100 detector |
Other groups use sodium iodide crystals and look for the scintillation light caused by the ionized electrons or search for crystal dislocations in samples of billion year old mica. Another possibility is to record the recoil of an ionized atom in a gas using drift chamber techniques.
Now particle physicists are proposing a new way to detect dark matter using the molecule of life - www.sciencenews.org
3.Indirect detection of WIMPs
The idea is that if the halo is made of WIMPs, then these WIMPs will have been passing through the Earth and Sun for several billion years. Since WIMPs will occasionally elastically scatter off nuclei in the Sun or Earth, they will occasionally lose enough energy, or change their direction of motion enough, to become gravitationally captured by the Sun or Earth. The orbits of such captured WIMPs will repeatedly intersect the Sun (or Earth) resulting in the eventual settling of the WIMPs into the core. As the number density increases over time, the self annihilation rate will increase. Since ordinary neutrinos can result from WIMP self-annihilation, one predicts a stream of neutrinos coming from the core of the Sun or Earth. Neutrinos easily escape the Solar core. The detectors on Earth capable of detecting neutrinos coming from Sun or Earth have operated for some time. The energy of such neutrinos is roughly 1/2 to 1/3 of the WIMP mass, so these neutrinos are much higher energy than the MeV scale solar neutrinos from nuclear reactions that have already been detected. The higher energy of these WIMP annihilation neutrinos makes them easier to detect than ordinary solar neutrinos and somewhat compensates for their much fewer numbers. Thus the presence of a source of high-energy neutrinos emanating from the centers of the Sun and Earth would be taken as evidence for WIMP dark matter.
For more info, please visit - http://www.snowmass2013.org.
Another candidate is axions. There are other possible dark matter candidates which do not fit into the WIMP framework. The axions arise from attempts to explain why the strong interaction seems to obey a certain symmetry called "CP symmetry". Among other things, CP symmetry would prevent the neutron from having a large electric dipole moment - without it, it's very hard to understand why such a dipole moment has not yet been detected. The best explanation for this, is called "Peccei-Quinn symmetry", and predicts a new light neutral particle called the axion.
For more info, please visit - physicsworld.com
Some astrophysicists are exploring the possibility of dark matter could lie inside the extra dimensions of the 'string theory'. The string theory adds six new dimensions to the four usual ones (three dimensions of space and one dimension for time) and would place the dark matter in these new dimensions which are inaccessible for us; explaining why it would not be detectable. The electromagnetic and nuclear forces (both strong and weak) would be confined in our four dimensions and could not leave them. If that's the case, there could be accompanying particles called Kaluza–Klein particles that account for dark matter. However, these would be even harder to detect.
For more info, please visit - alextorex.wordpress.com
Finally Massive astrophysical Compact Halo Objects( MACHOs )are non-luminous objects that make up the halos around galaxies. MACHOs are thought to be brown dwarf stars, neutron stars, white dwarfs, black holes. Unlike others, MACHOs are made of baryonic matter but, they don,t emit any light of their own so they are hard to detect. MACHOs can be found with gravitational lensing or gravitational microlensing. But there’s not enough ordinary matter (protons, neutrons, electrons) to account for the measured amount of dark matter so MACHOs are somewhat ruled out as a candidate. As we already know they are focusing more on WIMPs.
72% of the universe contains 'Dark energy'
Dark energy is trying to rip our universe apart.
 |
| Edwin Hubble |
Swan. This they combined with measurements of the 'red-shifts' of the spectral lines from stars in the galaxies.
In 1929, Hubble and Humason found that the galaxies were generally red-shifted and so were on average receding from us. They found that the amount of red-shift from a galaxy was roughly proportional to its distance from us, that is the galaxies more farther away were moving away with more speed and the galaxies near to us were moving relatively less slowly. So our universe is expanding (This discovery also gave rise to the 'big bang').
The mathematical equation for Hubble's law is as follows:
v = H0D
The velocity of recession of galaxies observed from earth (V), expressed in km/s, is directly proportional to distance (D), measured in megaparsecs or Mpc, where H0 is the value of the Hubble constant at the time of observation in the history of the universe. (A megaparsec is 3.26 million light-years). Currently the value for Hubble's constant is 69.32 ± 0.80 (km/s)/Mpc.
The velocity of recession of galaxies observed from earth (V), expressed in km/s, is directly proportional to distance (D), measured in megaparsecs or Mpc, where H0 is the value of the Hubble constant at the time of observation in the history of the universe. (A megaparsec is 3.26 million light-years). Currently the value for Hubble's constant is 69.32 ± 0.80 (km/s)/Mpc.
So after Hubble found out that universe was expanding, it was a very popular notion that some day our universe will stop expanding and as matter attracts matter through gravity, the gravity will the crush the universe onto itself. It's called 'the big crunch'.
Until in 1998 Adam Riess, of the Space Telescope Science Institute and Johns Hopkins University in Baltimore, led one of the first studies to reveal the presence of dark energy but, discovered something perhaps more profound than the expanding universe. By observing the cosmological red-shift of 42 different type Ia supernova it was discovered that the universe was actually accelerating or is accelerating. It was not only expanding, but that the rate of expansion was getting larger. Something was making the universe to expand faster, some kind of invisible force.
So the force was given the name of 'dark energy' Dr. Michail S. Turner. It's dark because we can't see it. Dark matter is one of the greatest mysteries out there today. Let's discuss some of the theories for it exsistance.
Candidates for dark energy |
| Source - HowStuffWorks |
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| Adam Riess |
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| Michail S. Turner |
The 1st candidate should be the 'cosmological constant'( otherwise known as 'vacuum energy' ) from General relativity its self. When Einstein solved his equation of general relativity for the 1st time, he found out that the universe was not static, in fact it was either expanding or contracting. He didn't liked the idea of a expanding universe so Einstein himself added a cosmological constant to his equations of general relativity to counteract the effect of gravity, that is, a constant which counteracts this effect. Friedmann, a Russian mathematician, realized that this was an unstable fix, like balancing a pencil on its point, and proposed an expanding universe model, now called the 'Big Bang theory'. Hubble's study of nearby galaxies showed that actually the universe was in fact expanding.
Einstein regretted, viewed the cosmological constant term as his "greatest mistake".
In 1980 it was put forward as a candidate for the repulsive force that produced the enormous exponential expansion, called 'inflation' of the universe.
Today it is proposed as a candidate for dark energy itself, which is responsible for driving the acceleration of the universe. The cosmological constant is the property of empty space itself.
If you remove all the particles and radiations of a small part of space, then you could call it an 'empty space' but, that does not mean that empty space is 'nothing'. It's not true. In physics because of the 'quantum mechanics' and 'special relativity' in very very small scales, empty space is a bubbling, boiling soup of 'virtual particles' that pops in and out of existence in a time scale so short that you can't see them. These "virtual pairs" consist of one particle with a negative charge and one with a positive charge. They exist for only a tiny fraction of a second before they collide and annihilate each other in a tiny burst of energy. These energy bursts creates ripples of energy in space-time fabric which may be driving the galaxies apart. So empty space has it's energy after all. So 'nothing' is actually something.
In the early universe, everything was closely packed so there was less 'space' fabric existed. The gravity was dominant. The cosmological constant is a property of empty space itself as universe grew over time, the gravitational pull weakened between structures and the cosmological constant dominated the growth of the universe.
But the problem is.
In Einstein’s equation, the cosmological constant is equivalent to an energy density in a vacuum, that is, a space devoid of matter. By equating this density to the density of the zero point energy that is left in a volume after you remove all its particles, you obtain a number that is 120 orders of magnitude higher than what is observed. Such a high value would result in a universe that would so rapidly inflate that there would never be enough time for galaxies to form. So simply the predicted value and the observed value doesn't match. The biggest problem with 'cosmological constant'.For more info, please visit - www.csicop.org
Another candidate for dark energy is is a scalar field called 'quintessence'. It is also proposed as a fundamental 'fifth force', a cosmic chameleon. It is time dependent that is it grows with time ans it is dynamic. So it's some what popular over the cosmological constant( doesn't change with time ).
Quintessence is a quantum field with both kinetic and potential energy. Depending on the ratio of the two energies and the pressure they exert, quintessence can either attract or repel. There are several hypotheses of quintessence fields, and in each one, each with a different outcome for the future of the universe.
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| Image credit: NASA / Chandra X-ray observatory. |
Another candidate is gravity itself. Einstein's general relativity predicts gravity as warp in the fabric of space-time and we take it for granted. But relativity only holds good for the universe on large scales, on the smallest scales, quantum gravity takes over. Relativity also breaks down in the presence of the strongest gravitational fields, like those at the center of a black hole, called the singularity.
And now we know that something called the 'dark matter', which constitutes 23% of the universe has much strong gravitational effect than the normal matter. The existence of dark matter is one of the key points in the evolution of our universe. Without it everything would have fallen apart.
And now we are becoming aware of something called the 'dark energy',which constitutes 72% of the universe and is accelerating the expansion of the universe. It is producing some 'anti-gravitational' effects.
So maybe there is more to the concept of 'gravity'.
For more info, please visit - news.nationalgeographic.co.in
If you didn't find the above link useful then please try - arxiv.org
So was dark energy always around from the beginning? Did the expansion of our universe ever slowed down? what is going to happen if the rate of expansion of the universe keeps increasing?
The Baryon Oscillation Spectroscopic Survey (BOSS) - part of the Sloan Digital Sky Survey (SDSS-III), relying on data from distant quasars have enabled physicists to produce a detailed 3D "map" of the early universe a whopping 11.5 billion years ago.
Boss measures the Baryonic Acoustic Oscillation (BAO). BAO is the result of the pressure waves or ripples propagating through the universe from it's most earliest phases( from 30,000 years after the big bang ) when everything was a very hot and dense soup of baryons and photons.
BOSS measurements indicate that dark energy was not always the superior force. It indicates that before a critical point 11 billion years ago, the gravitational force was dominant and the universe was slowing down.
If the acceleration continues then the fabric of space-time will be stretched to such an extent that it will be torn apart and the matters will be disintegrated. It is called the 'Big Rip'.
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| The dark energy camera |
- type 1a supernova
- Baryon acoustic oscillation (BAO)
- Gravitational lensing
- Growth of galaxy clusters over time
For more info, please visit - www.theverge.com
To solve the mystery of the dark matter and dark energy, mathematicians are also stepping forward. Two mathematicians offer a unified theory of dark matter and dark energy by altering Einstein's field equations. They suggested that the laws of conservation of energy and momentum in space-time is valid only when normal matter, dark matter and dark energy are all considered. For normal matter only, the laws will not hold good.
For more info, please visit - newsinfo.iu.edu
The day when the greatest minds will be able to understand what dark matter and dark energy exactly are. That will be the greatest victory for man-kind.
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