Our universe is dominated by matter. Sure, there is dark matter and dark energy, but things like stars, planets and people are made of matter. Protons, electrons, neutrons and such. But matter seems to come in pairs. For every electron created, an antimatter positron is created. For every proton that appears, so does an anti-proton. Since our universe is dominated by matter, what if there is another universe dominated by antimatter? What would an antimatter universe look like? 

The basic difference between matter and antimatter is that they have opposite charges. A proton has a positive charge, while an antiproton a negative one. Positively charged positrons are the antimatter version of negatively charged electrons. What’s interesting is that the signs of electric charge are a fluke of history. We could have assigned a positive charge to electrons and a negative one to protons. There’s nothing special about choosing one or the other. So you might think that an antimatter universe would look exactly like our regular one. But matter and antimatter have subtle differences.

One of the main differences has to do with neutrinos. Neutrinos don’t have any charge, so if the sign of charge were the only difference between matter and antimatter, “antimatter” neutrinos would be identical to “matter” neutrinos. But it turns out they are slightly different. Neutrinos have a property called helicity, which describes whether they spin to the left or the right as they travel through space. Matter neutrinos have left-handed helicity, while antimatter one have a right-handed helicity. That might not seem like a big deal, but in 1956 Chien-Shiung Wu looked at the radioactive decay of cobalt-60 atoms. She found that left-oriented and right-oriented atoms decay at different rates. Since handedness is different between matter and antimatter, the two might decay at different rates. This might be the reason why we don’t seen lots of antimatter in the universe.

But suppose there was an antimatter universe that had lots of anti-hydrogen and anti-helium after its big bang, just as our early universe had lots of hydrogen and helium. It would seem reasonable that these could fuse to heavier antimatter elements in the cores of antimatter stars, and this could produce antimatter planets and perhaps even antimatter life. What would these creatures see when they look up into their night sky?

In this case we know it would look much like our own night sky. Recently we’ve been able to produce anti-hydrogen, and we have looked at the type of light it produces. We found that anti-hydrogen produces the same kind of light as regular hydrogen. So an antimatter Sun would emit the same light as our Sun. Light would reflect off an antimatter moon just as it does our Moon, and our antimatter cousins would see a sky filled with stars, nebulae and planets, just like we do.

Of course all of this is based upon the assumption that antimatter would collapse under gravity to form stars in the first place. We think that should be the case, but what if antimatter also had anti-mass? What if anti-atoms gravitationally repelled each other? In that case, an antimatter universe would never form stars or galaxies. Our antimatter universe would simply be filled with traces of anti-hydrogen and anti-helium, and nothing would ever look up at the cosmic sky.  While we think antimatter has regular mass, we haven’t created enough of it in the lab to test the idea. For now we can’t be sure.

So it is quite possible that an antimatter universe would look nearly identical to our own. But it could be that an antimatter universe would be nothing but cold gas. It’s even possible that the radioactive decay of antimatter is so different from that of matter, that an antimatter universe can’t even exist.

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In astronomy we study distant galaxies by the light they emit. Just as the stars of a galaxy glow bright from the heat of their fusing cores, so too does much of the gas and dust at different wavelengths. The pattern of wavelengths we observe tells us much about a galaxy, because atoms and molecules emit specific patterns of light. Their optical fingerprint tells us the chemical composition of stars and galaxies, among other things. It’s generally thought that distant galaxies are made of matter, just like our own solar system, but recently it’s been demonstrated that anti-hydrogen emits the same type of light as regular hydrogen. In principle, a galaxy of antimatter would emit the same type of light as a similar galaxy of matter, so how do we know that a distant galaxy really is made of matter? 

The basic difference between matter and antimatter is charge. Atoms of matter are made of positively charged nuclei surrounded by negatively charged electrons, while antimatter consists of negatively charged nuclei surrounded by positively charged positrons (anti-electrons). In all of our interactions, both in the lab and when we’ve sent probes to other planets, things are made of matter. So we can assume that most of the things we see in the Universe are also made of matter.

However, when we create matter from energy in the lab, it is always produced in pairs. We can, for example, create protons in a particle accelerator, but we also create an equal amount of anti-protons. This is due to a symmetry between matter and antimatter, and it leads to a problem in cosmology. In the early Universe, when the intense energy of the big bang produced matter, did it also produce an equal amount of antimatter? If so, why do we see a Universe that’s dominated by matter? The most common explanation is that there is a subtle difference between matter and antimatter. This difference wouldn’t normally be noticed, but on a cosmic scale it means the big bang produced more matter than antimatter.

But suppose the Universe does have an equal amount of matter and antimatter, but early on the two were clumped into different regions. While our corner of the Universe is dominated by matter, perhaps there are distant galaxies or clusters of galaxies that are dominated by antimatter. Since the spectrum of light from matter and antimatter is the same, a distant antimatter galaxy would look the same to us as if it were made of matter. Since we can’t travel to distant galaxies directly to prove their made of matter, how can we be sure antimatter galaxies don’t exist?

One clue comes from the way matter and antimatter interact. Although both behave much the same on their own, when matter and antimatter collide they can annihilate each other to produce intense gamma rays. Although the vast regions between galaxies are mostly empty, they aren’t complete vacuums. Small amounts of gas and dust drift between galaxies, creating an intergalactic wind. If a galaxy were made of antimatter, any small amounts of matter from the intergalactic wind would annihilate with antimatter on the outer edges of the galaxy and produce gamma rays. If some galaxies were matter and some antimatter, we would expect to see gamma ray emissions in the regions between them. We don’t see that. Not between our Milky Way and other nearby galaxies, and not between more distant galaxies. Since our region of space is dominated by matter, we can reasonably assume that other galaxies are matter as well.

It’s still possible that our visible universe just happens to be matter dominated. There may be other regions beyond the visible universe that are dominated by antimatter, and its simply too far away for us to see. That’s one possible solution to the matter-antimatter cosmology problem. But that would be an odd coincidence given the scale of the visible universe.

So there might be distant antimatter galaxies in the Universe, but we can be confident that the galaxies we do see are made of matter just like us.

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Hydrogen is the most abundant element in the Universe. It consists of a single proton paired with an electron. Since the proton and electron are bound together, the electron must reside in particular energy states. When the electron transitions from a higher energy state to a lower one, it releases light with a specific color. Each energy transition for the electron corresponds to a particular color, and together they form the emission spectrum of hydrogen. All atoms have nuclei of protons and neutrons bound to electrons, and the resulting emission spectra allow us to determine what makes up distant objects. It’s one of the most powerful tools in astronomy.

One of the ways we distinguish particles is by their charge. Protons are positively charged, while electrons are negatively charged. In 1932 Carl David Anderson discovered a particle that had the same mass as an electron, but with a positive charge. Later it was discovered that protons also had a twin with the same mass, but negatively charged. Such charge-reversed particles came to be known as antimatter. We found that matter and antimatter could annihilate each other to produce intense gamma ray light, and intense energy could create pairs of particles consisting of one matter and one antimatter particle.

In principle, from basic antimatter particles we could create anti-atoms and anti-molecules. If an anti-proton is bound to a positron (anti-electron) it would create anti-hydrogen. According to our understanding of physics, anti-atoms should be identical to their matter twins except for the reversal of their charges. The positrons of anti-hydrogen should be quantized into the same specific energy states as electrons in hydrogen, and when they transition between energy states they should create the same emission spectrum as hydrogen. At least that’s the theory. Proving it is a much harder challenge.

Although we’ve been creating antimatter in the lab since the 1930s, it wasn’t until 1995 that we were finally able to create anti-hydrogen. That’s because the antimatter we create tends to form with lots of kinetic energy, and getting a positron and anti-proton to slow down enough to bind together is difficult. Once they do form anti-hydrogen we face a second challenge, specifically that anti-hydrogen, like hydrogen, is electrically neutral. When particles are charged we can easily push them around with electric and magnetic fields. It’s harder to to contain neutral antimatter.

But recently we’ve been able to create and hold hundreds of anti-hydrogen atoms for more than 15 minutes. That might not seem like much, but it means we can finally start doing some experiments on anti-hydrogen. In a recent experiment, anti-hydrogen atoms were bombarded with laser light. The positrons of those atoms absorbed some of the light, putting them in excited states. After a bit the positrons transitioned back to a lower energy state and released light of their own. It’s the first case of light being emitted by anti-atoms in the lab. The team that achieved this also measured one emission line of anti-hydrogen, and found that it was the same as that of regular hydrogen.

Although this is only a basic first test of light from anti-hydrogen, it’s a pretty significant achievement. As we’re able to create and store more anti-hydrogen for longer periods of time, we’ll finally be able to test whether anti-hydrogen has subtle differences in its spectrum that points towards new physics through the looking glass.

Paper: M. Ahmadi, et al. Observation of the 1S–2S transition in trapped antihydrogen. Nature (2016) doi:10.1038/nature21040

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An atom is comprised of dense nucleus of protons and neutrons surrounded by a diffuse cloud of electrons. Since an atomic nucleus is held together by the strong force, similar to the way gravity holds stars and planets together, you would think nuclei would be basically spherical. This isn’t always the case, and a few such as barium-144 are even pear shaped. This has to do with a subtle way the Universe works, which could explain one of its biggest mysteries. 

As Emmy Noether demonstrated in the early 1900s, symmetry is connected to the basic laws of physics. The presence of certain symmetries in the world is a property of certain unchanging quantity in the Universe. Noether’s theorem is an elegant demonstration of this connection, and is central to most modern physical theories. Noether showed us how some of the most powerful symmetries are those connected to space and time.

Take, for example, mirror symmetry, also known as parity. When you look in a mirror, the image you see is reversed. If you hold up your right hand, your mirror image would seem to hold up its left. Mirror image clocks would appear to move counter clockwise. So imagine a mirror universe. Not the evil Spock kind of mirror universe, but simply one in which the parity of everything is reversed. Americans would drive on the left side of the road, the Sun would rise in the west and set in the East, but fundamentally nothing would change. It would be no different than if we simply decided to switch the meanings of right and left.

The basic idea of the Wu experiment. Credit: Wikipedia user nagualdesign.

At least that would be the case if parity symmetry were true. While usually things are symmetrical under a switch of parity, there are some cases were parity is violated. This was first demonstrated by the Wu experiment in 1956. Chien-Shiung Wu looked at the radioactive decay of cobalt-60 atoms. If parity was conserved, then mirror image decay experiments should behave in exactly the same way. What she found was that mirrored experiments of cobalt-60 decayed in opposite directions. Since radioactive decay is driven by the weak nuclear force, this meant the weak force violates parity.

Another symmetry is related to electromagnetic charge. In our Universe protons have a positive charge while electrons have a negative charge. Charge symmetry considers what would happen if these charges were reversed. Since antimatter particles have the opposite charge of their regular matter partners, this would be like replacing all matter with antimatter. Since positive charges interact with each other the same way as negative charges, you would think that charge symmetry would hold. After all, it’s why charge is conserved.

The helicity of neutrinos and anti-neutrinos. Credit: Universe Review

But it turns out charge symmetry can be violated in a subtle way, again connected to the weak interaction, specifically neutrinos. While neutrinos don’t have any charge, they do have a kind of rotation known as helicity. If charge symmetry were true, then matter and antimatter should produce neutrinos with the same helicity. But it turns out matter produces neutrinos of one helicity, while antimatter produces antineutrinos of the opposite helicity. So charge symmetry is violated as well.  For a time it was thought that the symmetries of charge and parity could be combined into a more general CP symmetry that would be conserved, but there are radioactive particles that violate it as well.

So what does any of this have to do with pear-shaped atomic nuclei? The shape of a nucleus is determined by the various interactions that occur between the protons and neutrons (and quarks) within the nucleus. If those interactions were CP symmetric, there shouldn’t be a pear-shaped nucleus like barium-144. By studying odd nuclei like barium-144, we can gain clues about the ways CP-symmetry can be violated.

What does this have to do with astrophysics? Remember that charge symmetry is connected to matter and antimatter. Because charge is conserved, when any particle of matter is produced through some physical process, a corresponding particle of antimatter must also be produced. In the early moments after the big bang, when matter was being produced for the first time, there should have been an equal amount of matter and antimatter. But what we see today is a Universe dominated by matter. The origin of this matter-antimatter asymmetry is one of the great unanswered questions of cosmology. It’s been proposed that a violation of CP symmetry could have produced more matter than antimatter, but the currently known violations are not sufficient to produce the amount of matter we see. If there are other avenues of CP violation hidden within pear-shaped nuclei, they could explain this mystery after all.

Paper: C. S. Wu, et al. Experimental Test of Parity Conservation in Beta Decay. Physical Review 105 (4): 1413–1415. (1957)

Paper: B. Bucher et al. Direct Evidence of Octupole Deformation in Neutron-Rich ¹⁴⁴Ba. Phys. Rev. Lett. 116, 112503 (2016)  arXiv:1602.01485 [nucl-ex]

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One of the basic assumptions of cosmology is that the universe is on average the same everywhere. That is, it is uniform and isotropic. This assumption seems to hold up quite well, but naturally there are studies done that look for violations of this assumption. Every now and then a research project will find what seems to be a violation of this assumption, but they generally don’t pan out when examined more closely. But there are aspects of the universe that aren’t symmetrical, and one of the biggest is known as matter/antimatter asymmetry (or baryon asymmetry).

When we look at the universe around us, everything seems to be made up of matter. But from particle physics we know that every type of matter particle (proton, electron) has an antimatter sibling (anti-proton, positron) with an opposite charge among other things. We can make antimatter particles in the lab, but they always form paired with regular matter. For every positron we create, for example, an electron is also created. So when the very first matter in the universe appeared soon after the big bang, one would expect an equal amount of antimatter to also be produced. One would think the universe should be half matter and half antimatter, but it isn’t.

There are lots of proposed solutions to this mystery, such as the idea that the early universe had small imbalances that were magnified by early cosmic inflation. In that way we just happen to live in a region of the Universe that’s dominated by matter. In other regions, far beyond the observable universe, there could be regions dominated by antimatter. But that idea isn’t very satisfying, so a more popular model is that some mechanism in the early universe must have made matter more common than antimatter. As strange as that seems, there is such a mechanism, and it’s known as CP violation.

Credit: Flip Tanedo

The CP in this case stands for charge parity, which are two symmetries in physics. Here charge is simply the sign of the charge, thus electrons have a negative charge and positrons a positive one. Charge symmetry means that universe made of matter and one made of antimatter should behave in the same way. Parity can be described as a mirror image. If you hold up your right hand while looking in a mirror, your image will hold up its left hand. Typically parity is represented by spin, where one is right handed, and the other is left handed. Parity symmetry basically means that if left and right were flipped in the universe, nothing should change. If you combine these two symmetries you get CP symmetry. This means that if you flipped matter with antimatter and left with right everything should stay the same. It might seem pretty obvious that it shouldn’t make a difference if we called electrons positive and protons negative, any more than if we decided to switch the meaning of left and right. But it turns out that it isn’t so obvious for the universe.

In 1964 it was found that a neutral particle known as a Kaon came in two types that were CP duals of each other. If CP symmetry were conserved, then these two types of Kaons should decay at the same rate. What we found was that the two Kaon types decay at slightly different rates. The difference between the two is only about 3 parts in 1000, but it isn’t zero. Fundamentally what this means is that nature distinguishes between matter and antimatter on a cosmic scale. They aren’t simply reversible. So it’s possible that the universe might have slightly favored matter over antimatter in the early universe.

While we know that CP violation occurs, the known mechanisms by themselves aren’t enough to account for the matter/antimatter asymmetry seen in the universe. For the model to work, there must be some mechanism beyond the standard model that produces a large enough effect. It’s a mystery we still haven’t solved.

At this point all we can do is keep researching, and see if the universe tips its hand.

Paper: J. H. Christenson, et al. Evidence for the 2π Decay of the K₂⁰ Meson System. Physical Review Letters 13: 138. 1964

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I’ve been preparing for an intro physics class Monday, and that means covering Newton’s laws of motion. Since it is an introductory class I don’t discuss the nature of mass too deeply. Essentially I tell my students that there is inertial mass, given by the second law of motion, and a gravitational mass given by Newton’s law of gravity. I then go on to say that since everything near the Earth falls at the same rate, the gravitational mass in the law of gravity must be proportional to the inertial mass in Newton’s second law. That is, the two masses are equivalent, which is the heart of the equivalence principle.

But things are never quite as simple as they seem, and the concept of mass is no exception. In Newtonian physics there are not two types of mass, but three. There is the inertial mass, which determines the acceleration due to an applied force; there is the passive gravitational mass, which interacts with the local external gravitational field; then there is the active gravitational mass, which creates the external gravitational field in which other particles interact. Newton assumed that all three types of mass were one and the same, and it is generally assumed that Newton’s was correct, but nothing in general relativity requires it, and there is (as yet) no experimental evidence to validate it.

Acceleration “looks” like gravity.

When Einstein first proposed the principle of equivalence as a foundation to general relativity, his basic argument was that, without some external point of reference, a free-floating observer far from gravitational sources and a free-falling observer in the gravitational field of a massive body each have the same experience. Likewise an observer standing on the surface of a massive body and an observer which uniformly accelerates at a rate equal to the body’s surface gravity have identical experiences. Thus, the free-float and free-fall frames can be considered equivalent. In the same manner, the uniform acceleration frame and the surface frame are equivalent. This is known as the weak equivalence principle:

All effects of a uniform gravitational field are identical to the effects of a uniform acceleration of the coordinate system.

In order to formulate general relativity in terms of general covariance, Einstein later strengthened this argument to yield what is known as the strong equivalence principle:

The ratio between the inertial mass of a particle and its gravitational mass is a universal constant.

It is this latter principle which was experimentally validated by the classic Eötvös experiment, which determined that objects fall at the same rate regardless of their material consistency.

The strong equivalence principle does not require that all masses are equal. It only requires that an object’s inertial and passive masses are proportional. Although the equivalence principle says nothing about active mass, conservation of momentum does. If you apply conservation of momentum to two gravitationally interacting objects, you find that momentum is only conserved is if the active mass of an object is proportional to its inertial and passive masses. Thus in order to relate all three masses, we need not only the equivalence principle, but also the conservation of energy-momentum.

Matter vs. antimatter. Source LBNL.

The constants of proportionality can be wrapped into the gravitational constant, so it would seem we can simply follow Newton, set all three types of mass equal to each other and be done with it. There is, however, a catch. Although we can arbitrarily set the magnitudes of active and passive mass equal to each other, it is possible for them to be opposite in sign. In other words, if there was some weird type of matter that gravitationally repelled other masses, the equivalence principle and conservation of momentum would still hold true. The equivalence principle has been tested between regular matter, which requires all three masses to be the same. Since ordinary matter is mutually attractive we can say that Newton’s assertion is correct for matter.

But what about anti-matter? No one has been able to test this assumption, so we can’t say for certain. It is possible that active mass is negative for antimatter, which would mean it falls upward in a gravitational field. If that is the case, then although general relativity would still apply to regular matter, it wouldn’t apply to matter + antimatter. Since general relativity is a powerful and experimentally validated theory, it is generally assumed that Newton’s assertion would hold for anti-matter as well. But the only way to know for sure is to test it.

Recently we’ve been able to create usable quantities of anti-hydrogen, which will finally give us the chance to put antimatter to the test. It’s generally thought that antimatter will fall downward just like regular matter, but if it doesn’t, it will be time for some new ideas for gravity.

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