Adam: The Early Universe

April 1, 2020

Wednesday, April 1, 2020 Adam, brief report: The Early Universe The early universe Hey everyone, here is my presentation on the early universe. Will try to answer any questions asap. Adam

Q & A: Question from Clayton: 1. You say In the beginning that the 4 fundamental forces were combined, do you have any extra infor- mation on that? Such as how it would interact with the early universe at the time?

Answers: Adam: As for your first question, I actually only found rather hopeful text on the matter of the 4 forces. I read that it is coined “Theory of everything” but from my understanding there isn’t any headway on that. Than again, when reading about it, it does bring up string theory a lot so may Dr. Wheeler could shed some light on that. JW: There is evidence from particle accelerators that the strength of the strong, weak, and electromagnetic interactions changes with energy, and that at high enough energy these forces have the same strength. Even without such evidence it is tempting to suppose these interactions are different aspects of a single one, in the same way that electricity and magnetism are now unified into electromagnetism. Since the 1970s a great deal of work in high energy field theory has been devoted to finding such a unifying theory, a Grand Unified Theory, or GUT. Beginning in the 70s as well, theorists developed supergravity theories that include the other inter- actions. Containing all the known interactions – gravity, strong, electroweak – these theories are whimsically called Theories Of Everything, or TOEs. The only currently successful quantum TOE is string theory, which does succeed at giving a unified quantum theory of all known interactions, including gravity. Most predictions yet made from TOEs or GUTs focus on low energy; it is difficult to compute the ultra-high energy behavior expected in the early cosmos. In any case, at the earliest times, the Planck era, we must have a quantum theory of gravity such as string theory because the available energy is enough to spontaneously produce small black holes, which (being small) would quickly evaporate.

Questions from Clayton and Noah: Clayton In the lepton age particles and antiparticles are created, and most of the antiparticles are destroyed. Do you know if there were the same amount of particles and antiparticles made? And what happened to the matter/energy of the antiparticles after they were destroyed?

1 Noah On slide 6 you mention that most antiparticles were destroyed due to annihilation or instability. Could you expand more on what you mean by instability? Answers: Adam: As for your second question, Clayton, that leads into something called the Baryon asymmetry problem. The standard model nor general relativity explain this issue as both would assume an equal distribution of matter and anti matter. If you are still curious about the matter anti-matter problem I highly suggest looking up the Baryon asymmetry problem. Pretty fun to read about. I mostly said instability due to the proposition that there MAY have been more matter than anti-matter in the beginning. Probably poor wording on my part there.

JW: In known particle creation events, particle-antiparticle pairs occur so the expectation is equal numbers of particles and antiparticles arising from the quark plasma. However, a certain very few particles, notably the K meson, violate both Parity and Charge Conjugation. There is a fundamental theorem in quantum field theory called the CPT theorem, which states that the combination of three symmetries: Parity, Charge conjugation, and Time Reversal – must be conserved. If the CPT theorem holds then since the K meson decay violates CP it must also violate T. If time reversal, T, is violated then there will be slightly different numbers of particles and antiparticles in the early universe. It is assumed that these particles and antiparticles annihilated, leaving the slight excess of ordinary particles as the ones we see today. The energy of the annihilations would have heated the Universe and contributed to the overall expansion. Clayton I looked up the baryon asymmetry like Adam had suggested, and one of the theories was a university-antiuniverse pair where our universe ended up mostly particles and time travels forward. Then the antiuniverse ended up mostly antiparticles, and time travels backwards, I thought that was really interesting and wanted to know your thoughts on it JW I think that view makes a lot of sense! The best understanding of antiparticles in field theory is that they are normal particles traveling backward in time. It would be totally consistent for the Big Bang event, whatever it was, to send particles both directions in time, with the antiparticles the ones that went the other way. No surprise we don’t see them!

Question from Noah: Noah: How does the big bang theory predict the existence of the CMB? Also, aside from the redshift of galaxies and the existence of the CMB, what are the main sources of evidence for the big bang theory? Answers: Adam: The Big Bang theory predicts that the early universe was a very hot place and that as it expands, the gas within it cools. Thus the universe should be filled with radiation that is literally the remnant heat left over from the Big Bang, which is the CMB. Another piece of evidence for the Big Bang are primordial gas clouds. In 2011, astronomers found pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. These two clouds of gas contain no elements heavier than hydrogen and deuterium. Since the clouds of gas have no heavy elements, they likely formed in the first few minutes after the Big Bang. Other than that, the CMB and red shifted galaxies are the main pieces of evidence that I know of. JW: This is the most important point about the Big Bang.

Wikipedia Big Bang gives a clear list with detailed descriptions (below) and you should be familiar with these!

2 1. Hubble’s law and the expansion of space 2. Cosmic microwave background radiation 3. Abundance of primordial elements 4. Galactic evolution and distribution

5. Primordial gas clouds "[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its general features." Lawrence Krauss Observational evidence The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble’s law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by BBN. More recent evidence includes observations of galaxy formation and evo- lution, and the distribution of large-scale cosmic structures,[69] These are sometimes called the "four pillars" of the Big Bang theory. Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations. Remaining issues include the cuspy halo problem and the dwarf galaxy problem of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are currently unsolved problems in physics. Hubble’s law and the expansion of space Main articles: Hubble’s law and Expansion of the universe See also: Distance measures (cosmology) and Scale factor (cosmology) Observations of distant galaxies and quasars show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the ob- served objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder. When the recessional velocities are plotted against these distances, a linear relation- ship known as Hubble’s law is observed: v = HD, where v is the recessional velocity of the galaxy or other distant object, D is the comoving distance to the object, and H0 is Hubble’s constant, measured +1.3 km to be 70.4−1.4 s·Mpc by the WMAP. Hubble’s law has two possible explanations. Either we are at the center of an explosion of galaxies— which is untenable under the assumption of the Copernican principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Friedmann in 1922 and Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson, and Walker. The theory requires the relation v = HD to hold at all times, where D is the comoving distance, v is the recessional velocity, and v, H, and D vary as the universe expands (hence we write H0 to denote the present-day Hubble "constant"). For distances much smaller than the size of the observable universe, the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v. However, the redshift is not a true Doppler shift, but rather the result of the expansion of the universe between the time the light was emitted and the time that it was detected.

3 That space is undergoing metric expansion is shown by direct observational evidence of the cosmological principle and the Copernican principle, which together with Hubble’s law have no other explanation. Astronomical redshifts are extremely isotropic and homogeneous, supporting the cosmological principle that the universe looks the same in all directions, along with much other evidence. If the redshifts were the result of an explosion from a center distant from us, they would not be so similar in different directions. Measurements of the effects of the cosmic microwave background radiation on the dynamics of distant astrophysical systems in 2000 proved the Copernican principle, that, on a cosmological scale, the Earth is not in a central position. Radiation from the Big Bang was demonstrably warmer at earlier times throughout the universe. Uniform cooling of the CMB over billions of years is explainable only if the universe is experiencing a metric expansion, and excludes the possibility that we are near the unique center of an explosion. Cosmic microwave background radiation Main article: Cosmic microwave background The cosmic microwave background spectrum Measured by the FIRAS instrument on the COBE satel- lite is the most-precisely measured blackbody spectrum in nature. The data points and error bars on this graph are obscured by the theoretical curve. In 1964, Arno Penzias and Robert Wilson serendipi- tously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978 Nobel Prize in Physics. The surface of last scattering corresponding to emission of the CMB occurs shortly after recombination, the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly scattered from free charged particles. Peaking at around 372 ± 14kyr, the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent. The radiation is isotropic to roughly one part in 100, 000. In 1989, NASA launched COBE, which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 104, and measured a residual temperature of 2.726K (more recent measurements have revised this figure down slightly to 2.7255K); then in 1992, further COBE measurements discovered tiny fluctuations (anisotropies) in the CMB temperature across the sky, at a level of about one part in 105. John C. Mather and George Smoot were awarded the 2006 Nobel Prize in Physics for their leadership in these results. During the following decade, CMB anisotropies were further investigated by a large number of ground- based and balloon experiments. In 2000–2001, several experiments, most notably BOOMERanG, found the shape of the universe to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies. In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general. The Planck space probe was launched in May 2009. Other ground and balloon based cosmic microwave background experiments are ongoing. Abundance of primordial elements Main article: Big Bang nucleosynthesis Using the Big Bang model, it is possible to calculate the concentration of helium-4, helium-3, deuterium, and lithium-7 in the universe as ratios to the amount of ordinary hydrogen. The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for 4He/H, about 10−3 for 2H/H, about 10−4 for 3He/H and about 10−9 for 7Li/H.

4 The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for 4He, and off by a factor of two for 7Li (this anomaly is known as the cosmological lithium problem); in the latter two cases, there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by Big Bang Nucleosynthesis is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium.[84] Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than He3, and in constant ratios, too.[85]:182–185 Galactic evolution and distribution Main articles: Galaxy formation and evolution and Structure forma- tion Detailed observations of the morphology and distribution of galaxies and quasars are in agreement with the current state of the Big Bang theory. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then, larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently, appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures, agree well with Big Bang simulations of the formation of structure in the universe, and are helping to complete details of the theory. Primordial gas clouds In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. These two clouds of gas contain no elements heavier than hydrogen and deuterium. Since the clouds of gas have no heavy elements, they likely formed in the first few minutes after the Big Bang, during Big Bang Nucleosynthesis. Other lines of evidence The age of the universe as estimated from the Hubble expansion and the CMB is now in good agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of stellar evolution to globular clusters and through radiometric dating of individual Population II stars. The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift. This prediction also implies that the amplitude of the Sunyaev–Zel’dovich effect in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult. Future observations Future gravitational-wave observatories might be able to detect primordial gravita- tional waves, relics of the early universe, up to less than a second after the Big Bang.

Question from Brock and Christian: Brock: You mention that there were the 2 main theories. The Big Bang and the Steady State theory. Could you briefly explain a few main points on the steady state theory and why the CMB put the nail in the coffin for that theory? Christian: Is there any clarification you can give on the steady state model? It just seems strange that people were entertaining the idea of a steady universe after WWII when we had already observed the expansion of the universe in the 1920s. Is there any more to it than that?

5 Answers: JW: Hoyle’s Steady State model recognized the overall expansion, but postulated that the universe would nonetheless always look essentially the same. In particular, the average density of matter (about 1 hydrogen atom per cubic meter) shouldn’t change. In order to do this, Hoyle postulated that every now and then a Hydrogen atom would spontaneously appear, keeping the density constant. It didn’t require a very large production rate. One advantage of the idea is that it allows an infinite age to the universe. I suppose Hoyle found that comforting. The Big Bang, on the other hand, gives the universe as we know it a finite age of a bit over 13 billion years. The CMB is remnant energy from a previously hot, dense era. It’s current cool temperature and isotropy (and even the .0005 K anisotropy) is just as predicted by the Big Bang model, whereas the Steady State model has no way to predict its existence at all.

Also, see Galactic evolution above

Question from Christian: Can you expand on why exactly there’s a limit to how far back we can look? I understand the universe was opaque before recombination, but didn’t the CMB still exist and was interacting and wouldn’t it leave something for us to detect today?

Answers: JW: The CMB is the light from the Big Bang. All the light. What we’re looking at is a bright, hot plasma in every direction. Fortunately, it’s cooled off a lot! Recombination locks in the earliest time at which we can see the initial singularity. If we could see earlier, it would still be a uniform temperature isotropic gas, but hotter, but we can’t see earlier because light from remote places emitted before that got scattered before it got here. So the only light that reaches us now from distances of around 13 billion LY is light that was emitted after scattering stopped. That doesn’t mean we can never know about things earlier, but we need something other than light. This is one of the important features of gravitational waves. Gravitational waves produced in the initial cataclysmic whatever will not have been deflected significantly since they were produced. Only extremely large masses can deflect them, so they will travel uninterrupted from the very early universe.

Also, see CMB above

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