Antimatter-Antigrav

Why do we think antimatter is missing?

Every time we make matter, we also make antimatter, and we expect that the Big Bang also produced equal amounts of matter and antimatter.  But when matter and antimatter come into contact, they annihilate each other, converting mass into energy.  We know that the Earth is entirely made of matter, but what about the rest of the Universe?  We have walked on the moon and we have sent probes made of matter to other planets, so we know those are also made of matter.  In fact, we knew this before we went because meteors do not annihilate either when they strike the earth (so they must be matter) or when they strike other planets.  In fact, matter-antimatter annihilations give off so much energy that we would be able to see if the solar wind (made of matter particles) encountered any antimatter.  We do not see any large annihilation signals anywhere in the galaxy, so we are confident that all of the stars and gas clouds in our galaxy are also made of matter.

We can apply the same technique for looking for antimatter beyond our own galaxy.  All over the observable Universe we see galaxies and gas clouds colliding, but we never see them annihilating each other, so even though we cannot tell by looking if they are matter or antimatter, whenever they collide they have to be the same.  Is it possible that there are separate matter and antimatter domains?  There is no mechanism in the Standard Model of particle physics for creating these separate domains, but even if we assume such a mechanism exists, there should be interactions of gas at the boundaries.  These interactions would heat the gas, leading to more interactions.  We should be able to see the radiation from these interactions, so the fact that we do not see this radiation allows us to set a limit on the minimum size of the domains.  Cohen, De Rujula, and Glashow wrote a paper [1] about this, and through a complicated analysis they were able to set a lower limit on size of domains which is the size of the visible Universe.  This leads to the conclusion that the entire visible universe is made up of matter.

What happened to the Antimatter?

The Big Bang produced a tremendous amount of both matter and antimatter. Most of this annihilated back into energy, so it would only take a small asymmetry between matter and antimatter physics to produce an excess of matter after all the antimatter has annihilated.  In 1967, Andrei Sakharov spelled out the conditions necessary to create this “baryon asymmetry” [2]:

1. a process that violates “baryon number” conservation;
2. a process that violates C (charge) symmetry and CP (charge & parity (mirror)) symmetry;
3. out-of-thermal-equilibrium conditions.

In fact, we now know that physics does distinguish between matter and antimatter (the physics language for this is that CP symmetry is violated).  The problem is that this asymmetry is about 10 orders of magnitude (0.0000000001) too small to account for the observed excess of matter.

If we apply physics as we currently understand it, nearly all the matter that ever existed should have annihilated with antimatter.  The fact that a significant amount of matter still exists is a mystery, a mystery to which we owe our very existence!

Antigravity for Antimatter can solve this Mystery!

One solution to the mystery of the missing antimatter could be that  antimatter still exists, but is found in separate domains than the antimatter.  Just by looking, we cannot tell if any given galaxy is made of matter or antimatter.  So why do we think none of the galaxies we see are made of antimatter?  This is because we see galaxies that collide with each other, but we never see galaxies that annihilate each other, so all colliding galaxies must be the same, either both matter or both antimatter.  Turns out that we can extend this argument to gas clouds as well as galaxies.  If a matter gas cloud encounters an antimatter gas cloud, the annihilating radiation would heat up the gas and this would keep the interaction going.  Under normal gravity we would always be able to see the radiation from the annihilation interactions provided the sizes of the domains are smaller than the size of the visible universe [1].

However, if matter and antimatter repel each other through gravity, then neither galaxies nor gas clouds would ever collide with their anti counterpart.  Furthermore, tiny density fluctuations during the Big Bang would grow because of the gravitational force, providing a natural explanation for the separation of matter and antimatter into different domains.  It would only require tiny fluctuations, one part in a billion, to explain the observed matter density in the universe.

References

1. A. D. Sakharov, Violation of cp invariance, c asymmetry, and baryon asymmetry of the universe, JETP Lett., 5 (1967), p. 32.
2. A. G. Cohen, A. De Rujula, and S. L. Glashow, A matter-antimatter universe?, Astrophys. J., 495 (1998), pp. 539–549; arXiv:astro-ph/9707087.

Physicists have a big problem: two of our favorite theories, General Relativity (GR) and Quantum Mechanics (QM), are fundamentally incompatible with each other.  This is not a minor disagreement; they are infinitely incompatible, and the usual tricks physicists use to make infinities go away (renormalization) do not work here, because there are an infinite number of these infinities. And you thought you had a lot of problems…

There are many ways to express the incompatibility between GR and QM. General Relativity is a classical theory while Quantum Mechanics is, well, a quantum theory. All efforts to quantize GR to date have failed. Here is one way to understand the incompatibility: the quantum vacuum is not empty, it is filled with virtual particle-antiparticle pairs, and according to GR, these virtual particles should curve spacetime. In fact, they should curve spacetime so much that the Universe should have collapsed an instant after it was created. Obviously, it did not, so either there is some miraculous cancellation, or our understanding of GR and/or QM must be wrong. How miraculous? The observed curvature of spacetime is some 120 orders of magnitude smaller than expected. This has been called the worst prediction in physics!

However, if antimatter has antigravity, then virtual matter-antimatter pairs will have no gravitational charge, so they would not curve spacetime. Furthermore, if virtual pairs do not gravitate, it would invalidate the current indirect measurements that limit the difference in the gravitational charge between matter and antimatter because these measurements rely upon measuring the virtual antimatter content in matter.

Every direction we look we see radiation left over from the Big Bang, known as the Cosmic Microwave Background Radiation (CMBR). Those of us who are old enough to remember analog TV have seen some of this radiation in the “snow” we saw when we tuned to a channel between stations. 

The expansion of the Universe has stretched out the wavelength of this radiation so that it is now in the microwave part of the electromagnetic spectrum, but it was once much shorter, in the optical part of the spectrum and shorter wavelengths.  One remarkable aspect of this CMBR is that it nearly perfectly matches the radiation that is given off by a hot object in thermal equilibrium with its surroundings. 

This means we can measure the temperature of the Universe: it is currently 2.725 degrees above absolute zero. This temperature is the same to better than one part in a thousand in every direction. 

But this is a problem, because if General Relativity describes the expansion of the Universe, the Universe expanded so rapidly that different parts of the Universe could never have been in thermal contact with each other.  Another way to say this is that light emitted at the beginning of the Universe has not has not had enough time to cross the visible Universe because the expansion of the Universe has been so fast. Without the ability to interact, there is no reason different places should be at thermal equilibrium with each other.

The standard explanation for having the entire Universe at the same temperature while keeping General Relativity is to postulate a period of exponential expansion, called inflation. The basic idea is that the Universe first came into thermal equilibrium, then it inflated, then the inflation stopped, replaced by the slow expansion we see today. Since the entire visible Universe came from a very small region of space, everything is at the same temperature.

Unfortunately, there is a huge problem with this explanation. Inflationary theories appear to require fine tuning to get inflation started, and more fine tuning to get it to stop. In essence, we have just replaced one problem with two different problems. If we were comfortable with fine tuning, this fine tuning could have been used to make the initial temperature of the Universe the same everywhere without a physical mechanism, so it is not clear that shifting the fine tuning to inflation has really bought us anything other than the fact that the fine tuning is now less obvious.

Antigravity for Antimatter can solve this Mystery!

Fortunately, if we have a Universe with equal parts matter and antimatter which repel each other gravitationally, then we do not need inflation to make the Universe the same temperature everywhere we look. The reason is that the initial expansion of the Universe would be much slower, and this allows all parts of the visible Universe to be in causal contact with each other, allowing them to be in thermal equilibrium. The reason for the slower expansion is easy to see: with no net gravitational charge the expansion of the Universe is essentially coasting, whereas if everything is attracting everything else (as it is in the presently accepted concordance model) the initial expansion of the Universe needs to be much faster to overcome this attraction. Furthermore, this initial expansion rate has to be incredibly fine tuned; too small and the Universe rapidly collapses back upon itself, and too fast and the Universe flys apart so fast there is no time for any structure to form. In mathematical terms, we need the Universe to be balanced at Ω=1.

The mystery of Dark Matter is really the mystery of why the dynamics (motion) of galaxies and other larger structures do not match the predictions of General Relativity, our current theory of gravity.  Calling it the mystery of Dark Matter is to assume that Dark Matter is the solution, which might not be the case.  In fact, if antimatter has antigravity, it is possible that no Dark Matter is needed!

So let’s take a step back and see what observations tell us there is a mystery here.  In our current theory of gravity, the force of gravity falls off as the square of the distance from the source. For a large spherical body like the earth, the relevant distance is the distance from the center of the body rather than the distance from the surface, so we do not notice the change in the strength of gravity near the surface of the earth, but we can measure it. This means that planets further from the sun feel a weaker gravitational attraction to the sun than closer planets, so the planets must travel slower to stay in orbit around the sun. Neptune orbits the sun at a speed nearly 9 times slower than the speed of Mercury. But when we look at galaxies, this is not what we see! Rather than falling off with distance from the center of the galaxy, stars and gas clouds far from the center of galaxies orbit at nearly the same speed regardless of distance. This is an indication that the force of gravity is stronger than expected, and it is falling off at a different rate than our theory predicts. The same effect is seen in all large structures we observe in the Universe: the force of gravity is stronger than our theory predicts.

The generally accepted explanation for this discrepancy between the predictions of our theory of gravity and observation is that galaxies and other large structures contain substantial amounts of matter that cannot be seen apart from its gravitational interactions. This explanation is so prevalent that it is embodied in the name for the phenomena: Dark Matter. However, this is not the only possible explanation for the discrepancy, and in light of recent findings [1], it is neither the simplest nor the best explanation. The discrepancy can also be explained by modifying gravity itself. It has long been recognized that a simple modification to the gravitational force law known as Modified Newtonian Dynamics (MOND) fits the observations[2, 3]. 

It is interesting (and instructive!) to note that the “Dark Matter” problem is not the first case of “missing mass” in astronomy. In the 19th century, astronomers noted that the orbits of some of the planets did not exactly match their theoretical predictions, so they postulated the existence of unknown planets (missing mass) that were perturbing the observed orbits. In the 1840’s, Neptune was discovered at the location predicted by analyzing perturbations in the orbit or Uranus. At the same time, the planet Vulcanwas predicted to explain anomalies in the orbit of Mercury that did not fit the predictions of Newtonian gravity. But despite these anomalies and the “discovery” of Vulcan in 1860, the planet Vulcan does not exist. Rather, we now know that our theory of gravity was incomplete. General Relativity accounts for the anomalies in Mercury’s orbit, not the missing mass of the planet Vulcan. 

These historical precedents illustrate the question of whether Dark Matter is real or if it is an indication of a problem with our theory of gravity. Recent experimental and theoretical results suggest the latter. Experimental searches for Dark Matter continue to come up empty, and they have now excluded essentially all the possibilities that used to be considered the most likely places where Dark Matter would be found[4]. Of course, as possibilities were experimentally excluded, the places considered to be the most likely possibilities where Dark Matter would be found have moved, and it is impossible to prove that Dark Matter does not exist with negative results from direct searches since there is always the possibility that Dark Matter’s only interaction is gravitational, leaving no possibility of direct detection. This possibility is usually discounted because, with no additional couplings, there is no mechanism for creating the Dark Matter. Without a coupling between matter and Dark Matter, the “coincidence” that the amount of each in the universe differ by only a small factor is even more remarkable.

But the fact that direct searches have not found Dark Matter in the places we expected to find it is not themain reason to doubt its existence. The primary theoretical reason to doubt the existence of Dark Matter is that all galaxies that have been measured follow an empirical law that Dark Matter does not explain[1] (see figure 2). To put it another way, if Dark Matter were the explanation for holding galaxies together, there are many variations we expect to exist that we never see. Instead, in order to restrict the possibilities to those observed, the distribution of Dark Matter must be entirely constrained by the distribution of visible matter. Since there is no reason this should be true, it is more likely that the gravitational force from the visible matter is different from the prediction of General Relativity.

The best interpretation of the observations appears to be a modification of our theory of gravity rather than adding Dark Matter that we cannot see and have not been able to find, but how can we modify gravity without breaking all the measurements that confirm our current theory, General Relativity? Antigravity for antimatter appears to provide just the right modification. According to quantum mechanics, the vacuum is full of virtual matter-antimatter pairs that continually pop into existence and then disappear before they violate the uncertainty principle. If antimatter is repelled gravitationally from matter, then these virtual pairs will be gravitational dipoles. In a sufficiently strong gravitational field, these dipoles will polarize, and this will affect the gravitational field. We see this vacuum polarization with electric fields, and it is an essential part of the tremendous success of quantum electrodynamics (QED). In QED, the vacuum polarization screens charges, so that electric charges appear to be smaller than their bare charge. This can be measured in interactions at very small distances where the measured electrical charge is larger than at normal distances. But in QED, like charges repel and opposite charges attract, so while the vacuum polarization causes screening in QED, with gravitation it has the opposite effect because like charges attract. Also, because gravity is so much weaker than electromagnetism, the result of this anti-screening is only apparent over large scales such as galaxies. This anti-screening makes the force of gravity stronger, giving the appearance that there is more mass than what is visible (Dark Matter), but this apparent extra mass is really the polarized virtual particles in the vacuum and is fully determined by the gravitational field of the normal matter. The gravitational field from the normal matter is enhanced in such a way that it gives a mechanism for MOND [5, 6, 7]

References

1. [1]  Stacy S. McGaugh, Federico Lelli, and James M. Schombert. Radial acceleration relation in rotationally supported galaxies. Phys. Rev. Lett., 117:201101, 2016. Available from: http://link. aps.org/doi/10.1103/PhysRevLett.117.201101, doi:10.1103/PhysRevLett.117.201101.
2. [2]  M. Milgrom. A modification of the newtonian dynamics as a possible alternative to the hidden mass hypothesis. Astrophys. J., 270:365, 1983. Available from: http://adsabs.harvard.edu/ doi/10.1086/161130.
3. [3]  Benoˆıt Famaey and Stacy S. McGaugh. Modified newtonian dynamics (mond): Observational phenomenology and relativistic extensions. Living Reviews in Relativity, 15(1):10, 2012. Available from: https://doi.org/10.12942/lrr-2012-10.
4. [4]  Gianfranco Bertone and Tim M. P. Tait. A new era in the search for dark matter. Nature, 562:51–56, 2018. Available from: https://doi.org/10.1038/s41586-018-0542-z.
5. [5]  Luc Blanchet. Gravitational polarization and the phenomenology of MOND. Classical and Quantum Gravity, 24(14):3529, 2007. Available from:  doi:10.1088/0264-9381/24/14/001.
6. [6]  Dragan Slavkov Hajdukovic. Is dark matter an illusion created by the gravitational polarization of the quantum vacuum? Astrophys Space Sci, 334:215, 2011. doi:10.1007/ s10509-011-0744-4.
7. [7]  Dragan Slavkov Hajdukovic. Virtual gravitational dipoles: The key for the understand- ing of the universe? Physics of the Dark Universe, 3:34 – 40, 2014. Available from: http://www.sciencedirect.com/science/article/pii/S2212686414000077, doi: 10.1016/j.dark.2014.03.002.