NASA’s Fermi Gamma Ray Telescope has spotted something which should interest every physicist. Looking at the heart of our Milky Way galaxy, Fermi has unequivocally showed a bright gamma-ray glow. Scientists have then removed all known gamma-ray sources and, while it removes quite a bit of the contributing source, it still leaves a bit unaccounted for. We don’t know what’s causing this excess gamma ray glow. Given that gamma rays are some of the most energetic radiations known, it is unlikely that they are caused by some thermal event. The best explanation at the moment is that something unknown – some unknown particles – are annihilating each other and giving off these radiations. The question is then, what are these particles.
These particles ought to be quite heavy; the gamma ray emission hints at their mass. One very likely explanation for these particles is that they are Dark Matter particles. Humorously called WIMPs, short for Weakly Interacting Massive Particles, these heavy particles are likely candidates for Dark Matter (DM). In other words, the gamma ray lines seen by NASA’s Fermi telescope are because of DM annihilation.
Dark Matter 101
But what is DM you ask? DM is conjectured to be a type of matter beyond which we already know about, responsible for about 27% of the total mass-energy of the Universe. It was first hypothesized by Fritz Zwicky to explain why some galaxies can actually rotate as fast as they do without breaking apart. He surmised that there must be some invisible form of matter, which does not have any electromagnetic interaction, and thus doesn’t give off light, but are massive and, thus, can interact via the gravitational force. Today that conjecture stands on firmer grounds, with observations of known deviation from expected rotation speeds spanning thousands of galaxies. DM has been indirectly hinted at by many experiments such as the CoBE, WMAP and the recent Planck experiment, which all map out the distribution of Cosmic Microwave Background Radiation in our Universe. A host of other experiments also detect strong anomalies which can be easily explained away by the DM hypothesis.
In other words, we are quite sure that DM exists.
The clinching evidence would be to a actually detect it and one way is to let it annihilate each other into two known particles. These two particles then annihilate and produce some radiation which we can detect. The heavier the DM particles, the more energetic the final radiation; thus by knowing the final states, we can figure out the masses of the initial particles.
It is to be noted that no-one is jumping up and saying that DM has been found. While the evidence is highly suggestive, it’s not yet clinching, because, as most scientists like to say, not enough data has been collected. They would conservatively err on the side of mundane humility rather than make a mistake making an extraordinary claim.
“Tantalising hints” is a phrase high energy physicists have a love-hate relation with, and for good measure. Often all major discoveries remain tantalizing hints for a long time, creating a lot of confusion, generating a lot of debates and then either fade away into oblivion or become something so big that history just cannot ignore it. A recent demonstration of this phenomenon was given at the LHC during the hunt for the Higgs, where what remained as a ‘tantalising signal’ for months, grew steadily and offered the LHC physicists their first opportunity to say ‘Hurrah!’ A similar event may be afoot at the giant Super Cryogenic Dark Matter Search (SuperCDMS ) experiment, located deep inside a tunnel in the Soudan Mine in northern Minnesota. There is a ‘tantalizing hint’ of the detection of a particle which might be the elusive Dark Matter particle.
The recent tidings have excited physicists, as they have detected three events in the detector of a possibly weakly interacting massive particle (WIMP). However, they need more events. The surety of the discovery of these WIMPs is only 99.8 %, which amounts to a wee bit more than what physicists call 3-sigma confidence level. This is at the ‘tantalising hint’ level. At 4-sigma, it gets interesting and only at 5-sigma (which is a massive 99.9997% surety), do physicists say that they have a discovery.
The mass of the particle is somewhere about 8-10 GeV, which is about 8 to 10 times the mass of the proton. That’s low mass in the context and this particle, if present, should turn up in some of the other colliders and detectors, especially the LHC, in the near future.
Theorists are already busy at work figuring out how all of this fits into their theory. Can a supersymmetric theory, a theorist’s dream for a long time, accommodate the particle of mass 10 GeV? If so, which version and how has that version fared at the LHC?
For now, let this hint promote itself to the level of a discovery – we have seen too many tantalizing signals come and then disappear to be hasty.
The Universe seems to be just as queer as we could have supposed, or maybe less queer. The results from PLANCK, which came out in their first ever press conference, 15.5 months after the probe was launched, speaks of a Universe described almost entirely by what is known to be the Standard Model of Cosmology. In other words, there is nothing that should startle us, but a lot that should be enlightening and, frankly, quite exciting.
For cosmology virgins, Planck is a space probe launched into an orbit around Earth designed to pick up the radio and microwave radiations from the whole Universe. By charting out the whole sky, it creates a unique map, a map of the Universe, as seen from the microwave frequency regime, and not the optical regime that we are so used to. The Universe is extremely different in these frequencies from the star-filled Universe we know and love. But these frequencies tell a different tale – one of the early universe and what imprints of that we can see today. The story of Cosmology starts right at the beginning of the Universe, or rather, more accurately 10-32 seconds after it. The Universe underwent a sudden expansion phase, called inflation, and then stabilized, while continuing to expand. The radiation from the inflationary era have got both diluted (i.e. reduced in intensity) and ‘stretched’ (i.e. their wavelengths have increased, leading to a decrease in their energy) due to the expansion of the Universe which is continuing to this very day. So ‘cold’ have these radiations become that we need specialized probes to catch them, their temperature being just 3 K (i.e. 3 Kelvin above absolute zero). The redeeming fact about these strange low-energy waves is that they are everywhere – all over the Universe. They form a kind of ‘background’ radiation and are thus called ‘Cosmic Microwave Background Radiation’ (CMBR), the name being self-explanatory.
The theory goes that there arose minute quantum fluctuations in the radiation soup right after the inflationary phase. As the Universe expanded, these expanded, then gravity took over and clumped matter in these pockets of disturbed equilibrium. These manifest as galaxies or clusters we see today – the tiny quantum fluctuations have grown to giant scale. The imprint of these early fluctuations will be found in the radiation seen by Planck.
The Age of the Universe
First big observation – the Universe seems to be just a bit older than we thought it to be – about 70 million years older. So the official age of the Universe become 13.82 billion years, raised from the 13.75 (or 13.77) billion it was assigned earlier.
The constituents of the Universe
It seems that the known constitution of the Universe has changed slightly from previous estimates. The matter percentage of the Universe is slightly higher than what we had known. The constitution of the Universe is mostly unknown to us and we can only put in some percentages on the amount we know and the part we are ignorant about. For example, we know that matter forms a very small percentage, followed by dark matter which forms a large chunk and that dark energy – a strange form of energy responsible for the accelerated expansion of the Universe currently – forms the largest chunk of the matter-energy pie. The earlier estimates have been bettered by Planck which quotes 4.9% ordinary matter, 26.8% dark matter and the rest 68.3% as dark energy. This is a decrease in the estimate for dark energy from previous estimates and an increase in the estimate of normal matter and dark matter. This implies that we know a bit more about the Universe (we only know about the ordinary matter part) and that the Universe is expanding at a slightly less accelerated rate than what we thought. Our ignorance about most of the Universe is only slightly abated.
Closely related to this is the value of the Hubble’s Constant which Planck calculates to be 67.3 +/- 1.2 km per second per megaparsec. This is a big surprise from the earlier value of the Hubble’s constant 71.0 +/- 2.5 km per second per megaparsec. The lower value of the Hubble’s constant means that the Universe is expanding slower than earlier thought.
CMB Spectrum: Cosmic Fingerprint
The whole Cosmic Microwave Spectrum as predicted by theory matches that seen by Planck to very high precision. We see the Universe at very high angular width as well as very narrow width. What Planck says is that at high multipoles, corresponding to very narrow angular width, the data matches experiment exactly. At low values of multipoles, the error bars are large, but Planck has seen as much as can be seen.
No probe in the future will be able to see at finer resolution, since the limit on resolution is not placed by the instrument anymore, rather by the Universe itself as a whole. Our Universe not only seems perfect, it seems good at hiding possible imperfections as well.
Asymmetry and Anisotropy
Planck gives an asymmetry in temperature over the two hemispheres of the Universe. This is a startling find, but nothing absolutely new. It’s just that Planck has confirmed – at high resolution – something that WMAP had already hinted at.
An important point to be made here: The distribution of the fluctuations are exactly random, even though we might not feel it to be so. They pass all tests of randomness. Just to emphasize, the angular distribution of the fluctuations is really exactly random. The fluctuations could have been anywhere and they just happen to be where they are! What is important is that the amplitudes of the fluctuations are not random. The amplitudes – i.e. the real temperature of the fluctuations – are not random and one hemisphere seems to be on the whole hotter than the other. This wasn’t expected! Note that a small patch of sky could’ve been warmer than the other, but this is seeing a whole trend in the temperature – one side is colder and the other hotter – and we have nothing to explain that. In fact, our cherished notions of isotropy of space (I.e. cosmological phenomena and features don’t have a preferred direction) contradict this finding. We have to wait for a verdict on this. There is also a cold spot detected in the Universe – a region of space considerably colder than the other parts. No one knows why this is the case – is it just random or is there some forces at play there?
Let’s look at a couple of more topics, both being a bit more technical.
Neutrino masses and the limit on them:
Planck puts a stern limit on the sum of the neutrino masses – a value of 0.23 eV. This is at a 95% Confidence level and this result is completely consistent with the neutrino mass being zero. However, the phenomenon of neutrino oscillation says that neutrino mass cannot be zero, no matter how small. Planck also says that the number of neutrino species is 3 and no more, well almost. This rules out those elusive sterile neutrinos, the possible fourth species of neutrinos which don’t even interact via weak interaction and their effect is felt only through the gravity that they exert.
Spectral Index and Inflationary Theories
Inflation as a theory receives a major boost from these results. The simplest inflationary models predict that knowing the two-point correlation function would be enough since the whole spectrum of the fluctuations right after inflation can be modelled by a Gaussian. Planck reinforces that. The models also say that spectrum is scale invariant (or ‘conformal’) and Planck shows slight deviation from that. A quantity called ‘spectral index’, ns, quantifies the scale invariance. If ns=1, then the scale invariance is perfect, otherwise there is deviation. Planck gives the value of ns = 0.9603 +/- 0.0073. So inflation is also nearly as simple as we can imagine and all ‘complicated’ models of inflation can be ruled out. So Planck reveals our Universe in details we have never seen before. However, even after looking at the Universe this closely, we find that the Universe is indeed plain vanilla, with a couple of chocolate chips thrown in. People are calling it the MBU or Maximally Boring Universe. Is the Universe really less queer than we thought?
The Universe is accelerating, says a team of researchers led by Masamune Oguri, Kavli IPMU and Naohisa Inada at Nara National College of Technology, courtesy the data acquired by observing distant quasars. This is supplementary to the studies of distant supernovae, which also showed that the Universe’s expansion is accelerating, and for which the 2011 Nobel Prize in Physics was given. This study with quasars again shows that dark energy is definitely present, but we still don’t know what it might be.
Larger Data and more inferences
The data is derived from the Sloan Digital Sky Survey (SDSS), the huge collaborative experiment responsible for tracking about 100,000 quasars for nearly 10 years, with nearly 50 new quasars discovered in the last few years. Quasars are bright objects, believed to be formed, or at least fuelled, by the accretion of gas and dust by a supermassive black hole. The infalling material glows due to the enormous heat produced and can thus be detected from very far away. This makes them ideal for mapping the gravitational lensing occurring in the Universe.
Prof. Oguri, heading the study, says:
In 2011, the Nobel Prize in Physics was awarded to the discovery of the accelerated expansion of the universe using observations of distant supernovae. A caution is that this method using supernovae is built on several assumptions… Our new result using gravitational lensing not only provides additional strong evidence for the accelerated cosmic expansion, but also is useful for accurate measurements of the expansion speed, which is essential for investigating the nature of dark energy.
The Science of Gravitational Lensing
Gravitational lensing refers to the bending of light due to the presence of matter in the path of light, as explained by Einstein’s General Theory of Relativity. This process creates (at least) two identical images of one object, separated by a gap, thus the name ‘Cosmic Mirage’, referring to the similar process by which mirages on Earth are created.
The farther away the quasar, the greater its chances to be gravitationally lensed. Accelerated expansion of the Universe increases the distance of the quasar from us and thus the images also seem to separate (refer to figure above). This can be used to deduce how fast the quasars are receding from us. By plotting the velocity graph (velocity versus distance curve), we can see the deviation from the straight line expected from Hubble’s law, if the Universe was expanding at a constant rate. Sure enough, there is deviation and all the deviations fall on a curve, showing that it’s not just a mere statistical fluctuation or measurement error. The Universe is indeed accelerating! And this suggests that the estimates for dark energy are also not very off.
Dark Energy in Einstein’s Theory
Einstein’s theory of General Relativity allows for an expanding Universe without any extraneous assumptions. However, this expansion should have been at uniform speed. But it seems that the expansion rate is increasing. To get this prediction from Einstein’s equations, scientists tweak it a bit, adding a ‘cosmological constant’ term. This adds a bit of energy per unit volume of the Universe, contributing a lot to the entire energy of the Universe. By adjusting the sign of this extra term, the universe can be made to be accelerating.
In fact, it can be shown that we are exiting a phase dominated by matter, where the major contribution to energy comes from matter, and entering a phase dominated by the cosmological constant. This inevitably leads to accelerated expansion. We might not be able to see any galaxies in another 5-10 billion years, if the accelerated expansion of the Universe continues unabated.
The SDSS data also shows that treating dark energy as the cosmological constant is not such a wrong thing to do.
The future of the SDSS project is Planck and the SuMIRe projects. Both aim to study the distribution of cosmic dark energy and all this in the not-too-distant exciting future!
When galaxies form, they leave a lot of debris strewn around and this might reveal vital clues about the nature of dark matter, think scientists. Galaxies are believed to form by collision of smaller bodies of stars, which then equilibriate due to gravitational interactions. Even the Milky Way is believed to be surrounded by smaller galaxies, called satellites. A group of scientists looking at the Local Group, or a group of a few galaxies, including our Milky Way and the neighboring Andromeda, have found that the number of observed satellite galaxies is much smaller than expected from cold dark matter simulations.
These satellite galaxies have unusually low number of stars and can be made up of mostly dark matter. These dark satellites could only be seen by the way they bend light due to their gravitation.
Searching for Dark Satellites
Attempts to find dark satellite galaxies in the Local Group had yielded a candidate at a redshift of z= 0.222, which works out to about 3 billion light years, which is quite close by cosmological standards. However, the number was just not tallying up; out of a predicted number of 10,000 only 30 showed up. Scientists decided that the Local Group case might be an anomaly and looked further out.
In a paper published in Nature, by the team of Vegetti (MIT), Lagattuta (University of California, Davis), McKean (Netherlands Institute of Radio Astronomy), Auger(University of California, Santa Barbara), Fassnacht (University of California, Davis) and Koopmans (University of Groningen, Netherlands) report to have observed a dark satellite galaxy at a redshift of 0.881, which puts its distance at about 10 billion light years.
Using General Relativity to See!
The method is a beautiful consequence of Einstein’s General Theory of Relativity, the modern theory of gravity. It says that mass bends space-time and light rays will follow this bent path as they travel. This allows us to use a technique called gravitational lensing. The technique is quite simple in principle. A dark matter object will act as a lens, bending light as it travels through it. If you have a light-emitting galaxy behind a dark matter galaxy, the light will get distorted and we will be seeing an image of the galaxy at a position where it is not really there. A beautiful demonstration of this is manifested as an Einstein’s Ring.
Using the Keck-II telescope, one of these Einstein Rings was observed and a satellite galaxy was noticed at about 10 billion light years away, which has about the same mass as the Sagittarius galaxy. This is the first gravitational detection of a low-mass dark satellite galaxy at cosmological distances!
Exactly as predicted!!
The most exciting feature about this is that the mass calculations yield just the right mass for the galaxy to be consistent with cold dark matter simulations. It gives more credence to the hierarchical formation of stellar structures like galaxies as described by cold dark matter models.
The existence of this low-mass dark galaxy is just within the bounds we expect if the universe is composed of dark matter that has a cold temperature. However, further dark satellites will need to be found to confirm this conclusion
Ignorance may not always be blissful, but it is certainly exciting most of the time!
A manhole like plate with holes punched in a certain distribution may be the key to understanding the greatest problem facing cosmology at the moment. David Schlegel plans to investigate the nature of Dark Energy and its effect on galaxies using aluminium discs, 2,200 in all, all with a specific set of holes punched in through them. These can be used to view a part of the sky using the 2.5-metre telescope at Apache Point Observatory in New Mexico.
Looking at galaxies…
The scientists will then take spectrum from each of the galaxies. This will yield valuable information as to how fast they are moving away from us, giving hints about the effect of dark energy on these celestial congregations. The discs are to be used for looking at a particular section of the sky. Each hole, when looked through, allows the telescope to stare at only one galaxy. This will eventually collect data from as many as 1.5 million galaxies! The project began in 2009 and is all set to present its first set of data on the 11th of January.
David Schlegel, the principal investigator of the project, called Baryon Oscillation Spectroscopic Survey (BOSS), says:
The more galaxies we get, the better.
… And more than looking at galaxies
BOSS is doing more than just looking at galaxies and their recession speeds. Looking at galaxies and their distribution, the team has found signatures of clumping of galaxies. This is in contrary to the near-uniform distribution of galaxies that one might expect to see. This might be a remnant of a disturbance right after the Big Bang, whose signature just failed to wash away! In fact, it is believed that random quantum fluctuations, called Baryon Acoustic Oscillations, disturbed the uniform energy density distribution of the Universe, right after the inflationary phase, and this process seeded regions where matter could clump together. As more matter fell, gravity aided in the cascade-like process. These became galaxies. Whether there are fainter and less familiar structures contributing to the overall structure of the Universe, we just don’t know.
BOSS takes over from the previous WiggleZ survey, which surveyed about 240,000 galaxies, as compared to BOSS’s 1.5 million. The BOSS team is already planning to get BigBOSS. This will be the ultimate megaproject, sampling about 20 million galaxies and peering deep into the Universe and seeing older and older structures!
How wonderful that this should come at a time, when the public perception was freshly filled accelerated expansion of the Universe, given that the Nobel Prize in 2011 was given for findings on the accelerated expansion of the Universe, using Type Ia supernovae. BOSS, and BigBOSS, will definitely do better!
The Universe may not be as dark as previously thought, if an Italian mathematician is to be believed. A. Carati, an Italian mathematician, has made a bold proposition if faraway matteris allowed to interact with galactic matter, it can be shown that their gravitational effects reproduce the galactic rotational curves extremely faithfully! This rules out the need for any non-luminous, or dark, matter.
The Inception of Dark Matter
The problem of dark matter in case of galactic rotation curves is stated as follows. Astronomers have made theoretical predictions (using Einstein’s General Relativity) about the speed of the different parts of the galaxy and their distance from the center. When the velocity is plotted against distance, what we get is called a velocity curve. The problem is that while you expect a certain curve given the amount of luminous matter in a galaxy, what is directly observed is that the velocity curve deviates significantly from this, especially as one goes away from the center. Astrophysicists invoked the presence of dark matter’ or non-luminous matter to save Einstein’s gravitational theory.
This is what a typical curve looks like.
Enter the mathematician
Now, what Carati claims is that if faraway matter is allowed to exert gravitational forces, which, of course, it does, then the rotation curves for spiral galaxies can be explained away. The effect of matter in faraway galaxies is generally neglected in all calculations, since the galaxies are assumed to be uniformly distributed throughout the Universe. (This is the principle of isotropy in space, which is a guiding principle for Cosmology.)
The idea that galaxies might be arranged according to a very complex fractal structure was originated by Sylos Labini in 1998. Carati took up from this point, and making a few simplifying assumptions about the de-correlation of matter at large distances, he showed that the forces that this matter might exert is of the order of 0.2 cH0, H0 being the Hubble’s constant.
The crucial point is that this force is good enough to explain the anomalous rotation curves! And the fit is amazingly accurate.
If seeing is believing, or, at least, suspending disbelief we present the rotation curves for two spiral galaxies NGC 3198 and NGC 2403. Look at the expected curve and how well the observed points fit it!
True, this is an outlandish idea, but so is dark matter. The fact that General Relativity can produce such stunning verification of the experimental data is quite satisfying.
We don’t have any word on how this will solve many of the other puzzles that dark matter solves, mainly the Cosmic Background radiation distribution (WMAP data) and the observed gravitational lensing effects.
The mystery deepens. Scientists have found distribution of dark matter in dwarf galaxies, which is completely anomalous to the current theory of cold dark matter distribution in the Universe. The AMS observation from aboard the ISS has shown that dark matter is spread uniformly throughout the dwarf galaxy, whereas one would expect it to clump around the centre and then thin around towards the edges. This pattern, based on the prediction of the cold dark matter model, is seen in bigger galaxies like our own. Dwarf galaxies provide an exception and no one knows why.
What really is Dark Matter?
Dark matter refers to the attractive positive density matter that supposedly makes up as much as 23%-26% of the Universe, as compared to the 4% of visible matter (all stars, planets, galaxies everything that we know about!). The rest is dark energy and it is the energy of the vacuum, tending to tear to Universe apart making it accelerate its expansion. Dark matter was proposed by Fritz Zwicky and was initially invoked to explain the high speed of the outer spirals of a galaxy. They were spiraling too fast to be held by gravity of the visible matter. An invisible matter, interacting only through gravity had to be present. Gradually, it was recognized that dark matter could explain other things as well, like the observed cosmic microwave background radiation, unexplained gravitational lensing etc. Matter is believed to reside in the space-time trough created by lumps of dark matter.
The most successful version of dark matter has been the cold dark matter’ model, which says that the dark matter particles, whatever they may be, are moving very slowly with respect to light.
The Study and an Anomaly
The study involved observing the Fornax and Sculptor galaxies, which orbit the Milky Way. These are dwarf galaxies and are thought to be primarily made up of dark matter. What was expected was that the center would be rich in dark matter and then the distribution would thin out towards the edges. What was seen instead, was a uniform distribution, confounding scientists to no end. Matt Walker, the leader for this study conducted by the Harvard-Smithsonian Center for Astrophysics, says
Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data.
Dark matter distributions can be inferred from the motion of stars. The presence of matter curves space and matter particles follow curves on this curved space, called geodesics. The geodesics would be significantly modified by the presence of dark matter. This gives an estimate to the dark matter present. The team investigated the motion of about 2000 stars and found this anomaly.
Matt Walker says
After completing this study, we know less about dark matter than we did before
Dark matter may finally have been driven out of its hiding place. Scientists report seeing as many as 67 events of dark matter detection at the Cresst Experiment in Italy. This is the startling news coming out of the ongoing 7-day Topics in Astroparticle and Underground Physics Meetingin Germany. The announcement was made yesterday, i.e. on the 6th of September.
The events have been detected with a four-sigma confidence level, or with about 99.994% confidence. For the status of a discovery, the detection has to be at a five-sigma confidence level or at 99.99994% confidence. Thus, we may say that the signals are extremely strong, but not strong enough to warrant a discovery.
What is Dark Matter?
Dark Matter has been predicted to occupy 24% of the Universe, with ordinary matter occupying just 4-5%. The challenge in detecting dark matter is due to the fact that it interacts really weakly with matter (hence, its name). It was introduced into the theoretical framework to explain the rapid spinning of the galactic arms. It soon turned out that fine-tuned Dark Energy Models are needed to explain Cosmic Microwave Background Radiation and Gravitational Lensing.
Dark Matter particles must have mass. Thus, the constituent particles of dark matter are called broadly as Weakly Interacting Massive Particles’ or WIMPs. The Cresst experiment uses supercooled calcium tungstate crystals to search for dark matter particle. When a dark matter particle hits one of the crystals, it scintillates and the detectors can pick up that pulse of radiation and measure the energy of the collision.
While this is great news for most gravitational models, it doesn’t reconcile with the non-detection of dark matter events in the CDMS-II and XENON100 experiments, both of which are also searching for Dark Matter particles. The fact that the detection is not at a 5-sigma confidence level yet means that all hopes of reconciliation are not gone.
Dark Matter may finally have been smoked out of its hiding place, but we’re still not sure of that.
Reality, aren’t thou a heartless beast! A scientist from CERN, Dragan Slavkov Hajdukovic, has claimed that dark matter may be an illusion and that its supposed effect can be explained away by more known kinds of particles simple matter particles and their corresponding anti-particles.
Option 1: Dark Matter
A startling observation in astronomy set out the hunt for an unknown form of matter, just because the gravitating effect of such hypothetical matter could explain the observations. Galactic arms were seen to be moving too rapidly for them to stay attached to the rest of the galaxy, if Newton’s law of gravitation (or even Einstein’s General Relativity) is to hold. Scientists could save the situation by postulating the presence of a large amount of non-luminous or dark matter, interacting with the rest of the Universe only by the gravitational force. No candidate for this kind of matter was proposed and people have come up with various conjectures and models. Right now, we know nothing about the composition of dark matter. It is said that dark matter comprises 23% of the known matter of the Universe. 73% is dark energy and only the rest 4% is normal matter as we know it.
Option 2: Modified Newtonian Dynamics
There was another way to resolve the puzzle modify Newton’s law of gravity. This new school of thought, called MOND or MOdified Newtonian Dynamics, said that the power law dependence of the gravitational force on distance (which is inverse-square for Newton’s law) depends on acceleration. MOND doesn’t need any dark matter. The problem with MOND is that it doesn’t seem to work at all scales of acceleration. Inherent inconsistencies prevent MOND from taking over from Newton’s gravity law.
Option 3: Gravitational polarization of Quantum Vacuum
Now, Hajdukovic postulates another possibility that space itself might be repulsive. He says that if particles and anti-particles were to repel each other gravitationally, rather than universally attract, then it would solve the problem. Vacuum could be polarized, i.e. particles and anti-particles could be created at will, governed by the Heisenberg’s Uncertainty Principle. Now, if these were to act as gravitational dipoles that repel, then the problem of amplifying gravitational fields would be solved. He gives an example of a dielectric slab inserted inside a parallel plate capacitor as an example. In this case, the electric field within the dielectric would decrease. Hajdukovic argues that, if unlike charges repelled instead of attracting, the field would increase in strength. The same, he concludes, is true with gravity.
Hajdukovic put it precisely:
Concerning gravity, mainstream physics assumes that there is only one gravitational charge (identified with the inertial mass) while I have assumed that, as in the case of electromagnetic interactions, there are two gravitational charges: positive gravitational charge for matter and negative gravitational charge for antimatter
As alien as that might sound to a reader versed in Physics, mainstream labs are actually working to find out whether anti-matter repels matter gravitationally. Primary amongst these is the AEGIS experiment at CERN.
The proposal also has a very pleasing symmetry. Gravitational force is unlike the electromagnetic force, since electromagnetic forces both repel (like charges or magnetic poles) and attract (unlike charges/magnetic poles). If Hajdukovic is right, then even gravity will have a repulsive component, but this might mean that we have to take another look at Einstein’s equations of General Relativity.
There are many issues, however. Things like gravitational lensing is explained best by assuming dark matter than without it.
Hajdukovic published his paper Is dark matter an illusion created by the gravitational polarization of the quantum vacuum?in the journal Astrophysics and Space Science.