Where we left off last time... The Planck Era is pure speculation about topics that are being explored in detail today (gravity, string theory, etc.) The GUT era matches what physicists see in particle colliders but on the earth we only see the effects of 'unification' at high energy The particle era is well-understood except for asymmetry Matter and 'physics' (the fundamental forces) condense out of the primordial radiation field We think we can constrain the energy make-up of the Universe from its observed expansion and a General Relativistic 'metric' I've seen the evidence and solved the equations and matched observations to this cosmological models I've seen that it works and trust that it's the best way we have to describe the Universe. That doesn't mean you have to take my word for it I encourage you to ask me and Professor Olszewski questions, read about it.
Constraining the 'Universe we live in' with BBN We could have lived in a Universe with any amount of ordinary matter and any abundance of light elements relative to hydrogen Observations of the abundances of light elements tell us what occurred in the early Universe this is the only time deuterium, lithium, and such a large amount of helium could have formed Credit: WMAP
What problems does dark energy solve? Dark energy is one solution to the accelerating expansion of the Universe, where else can we apply it? There's a major problem in large scale structure and the cosmic microwave background that we haven't discussed. Take a look at the CMB Credit: WMAP Credit: COBE These two images actually show the exact same thing the difference is contrast
So the CMB is extremely uniform, so what? We started with the cosmological principle the Universe is homogeneous and isotropic. But it actually breaks down if the Universe is not in 'causal contact' with itself. Look at the Universe today, there are huge voids (between galaxies) and huge densities of material (the earth). It's not actually hard to make the Universe inhomogeneous, rather it's hard to do the opposite. This is hugely important we are always fighting entropy. The Universe would have to have been extremely small when the CMB formed to be so uniform. But our theory says that it wasn't. This is more than uniformity remember the Copernican principle 'we're average, we don't live in a special place in the Universe.' Looking up to see the cosmic microwave background is as improbable as finding planets just like earth with civilizations just like us around every star. This is called the Horizon Problem the Universe must have suppressed inhomogeneities (smoothed them out) and preserved its uniformity long enough for the CMB to be released before entropy could disarrange all of the matter.
Another Significant Problem This problem is more esoteric, dealing with the 'shape' of the Universe Without going in to too much detail, the amount of matter and energy in the Universe affects its 'shape' (in the same way that matter can bend spacetime). Too much matter and the Universe is curved such that it collapses in on itself, too little and it expands too rapidly and all the matter flies away too quickly We call this the Flatness Problem there's some special amount of 'curvature' based on the relative amounts of dark energy and matter that make the Universe 'flat.'
Cosmic Inflation Let's go back to the GUT era. The Universe is still in a 'phase' where the forces, matter, and energy are all unified. At some point, the Universe cools down to the point where the strong nuclear force can 'freeze out.' This goes back to the GR metric - think about it this way, the strong force is like water vapor. As it changes phases to water and then ice, it gives energy back to the Universe (boiling point, fusion point). This rapid increase in the amount of energy in the Universe causes a rapid expansion. This expansion is predicted to have the property of 'smoothing out' any inhomogeneities in the Universe until all that was left were small fluctuations on the sub-atomic level. Mathematically, we refer to the process that causes this expansion as the 'inflation field.' This field is identical to the cosmological constant, which gives us an origin for dark energy the period of decoupling the follows the GUT era
Cosmic Inflation One of the biggest successes of inflation is that it smooths out any offsets from curvature in the same way that it smooths out inhomogeneities Inflation is a rapid expansion of space in a small period of time, rather than magnifying the wrinkles in space we find, mathematically, that this process shrinks any wrinkles in the early distribution of matter in the Universe Credit: NASA, WMAP
Fast Forward to the CMB The Universe has formed light elements but it's still fairly uniform and hot (3000 Kelvin) From the simple assumption that the Universe still contained hydrogen and it was cooling, we know that the hydrogen must have been ionized One the Universe was cool enough, there was some period of time when the hydrogen started to recombine (all at once) Electrons 'block' the radiation, so the CMB streams away when hydrogen recombines The radiation is redshifted, so we know exactly how long ago the CMB was formed The CMB is like a 'snapshot' of the Universe from the time that it formed Credit: COBE, FIRAS, ARCADE
Again, what do we know about the CMB? It's extremely uniform up to a point, it's like a blackbody, it must have been formed a long time ago There are places where we see more or less radiation from the CMB, so it appears hotter or colder we think that these regions correspond to overdensities and underdensities Tiny fluctuations in density that were suppressed by the inflation field had time to grow into small, but noticeable overdensities and underdensities in the amount of matter in the Universe Credit: WMAP
You should know that this is the most useful way to analyze the CMB it's called a 'power spectrum' The peaks in the power spectrum correspond to the size scales at which the CMB varies the most
Big Picture The formation of the CMB is really a pivotal moment in the history of the Universe before the CMB matter and radiation are coupled so our model is simpler, we know that the Universe is very uniform, we can be very accurate about the conditions of the Universe The CMB is also the earliest point in the Universe that we can probe directly everything else we know about the Universe involves constructing models and testing them against 'tracers' We have to describe how the overdensities in the CMB turned into the structure we see today in the Universe How did the first galaxies and stars form?
The biggest change in the Universe since the time the CMB formed is fairly simple the Universe was almost entirely neutral and now the Universe is almost entirely ionized (we observe way more ionized hydrogen than neutral hydrogen) This is one of the most important guiding questions for anyone who studies galaxy evolution and the so-called 'highredshift Universe'
Reionization This problem is huge in magnitude but relatively straightforward we just want to match the 'state' of the Universe at different times to what we think is the rate at which reionization occurs So, what are the ways that the Universe can ionize all of this hydrogen Hydrogen is reionized at high energies (13.6 ev, 91.2 nanometers, this is right in the middle of UV wavelengths) So what produces a lot of high energy light, e.g. UV and Xrays? Stars, especially high-mass stars Active Galactic Nuclei (AGN) which are a type of massive, compact object that emit a lot of UV and X-ray radiation In general, the idea here is to look as far back into the early Universe as we can, add up all of the light from stars and AGN we can see, and see if it's enough to ionize all of the hydrogen in the Universe.
Reionization Almost all of the problems and limitations involved in studying reionization come from observation these are very distant, very faint objects Because we're looking at faint objects, we have to limit our observations to probing tiny patches of the sky and extrapolating that to the whole Universe (cosmological principle) A somewhat tricky idea: when we look for the furthest galaxies, we mostly see the brightest ones and these may not be representative of all galaxies (bias) Credit: Hubble Extreme Deep Field
Reionization: Does it work out? Short answer no. We haven't seen energetic objects in high enough numbers yet to ionize the Universe at the rate we predict it should happen Is this a problem that has to do with bias again? Maybe we can't detect a lot of small, faint galaxies. All of the energy in starlight could add up and ionize the Universe in the time that we have Is this a problem that has to do with one of our assumptions? Maybe the galaxies we're seeing in the early Universe aren't like the galaxies in the Universe today. That would make a certain sense, after all. There are more properties of the Universe that have changed since the Big Bang. Think back to the Big Bang and early nucleosynthesis all we made was some hydrogen, helium, and a few other light elements. Essentially no carbon, nitrogen, oxygen, etc. Stars in the early Universe started producing all of these other elements, but the 'first stars' must have been a lot different without any light elements
Population III stars Think back to Walter Baade and his two populations of Cepheid Variables what was the key difference between each population? Metal-enriched stars like the Sun are 'Population I' they formed later when the metal content of the Universe was larger Metal-poor stars are called 'Population II' we observe these stars and we guess that they must have formed earlier in the metal-history (a relatively good clock for the matter-dominated era) of the Universe Right when stars started to form, there must have been a population that contained no metals. They definitely exist(ed) For the purposes of a 'reionization history thought experiment' let's assume we can distinguish them metalpoor stars (Pop II) What would be different about these stars?
Population III stars It turns out that stars that form with virtually no metals in them tend to be slightly more massive than metal-rich stars (their 'initial mass function' has more high-mass stars) What do we know about high-mass stars? They tend to be way more luminous than low-mass stars. More precisely, a galaxy with Population III stars put will produce more ionizing radiation than a modern galaxy Population III stars may help to solve other problems about the history ofthe early Universe forming massive black holes and enriching the Universe in a short period of time. All of these ideas (ionization, massive black holes, enrichment) turn on a central point we know what the Universe is like today, we think we know what it was like around the time of the CMB, but we can't describe how it got from point A to point B. Either we're wrong about what the early Universe was like or our current ideas about how the Universe evolved are wrong/incomplete. The second possibility is much more likely.
Population III stars Detecting individual stars in the early Universe is virtually impossible, so we have to look for 'signatures' of these stars instead Population III stars might produce a large amount of one type of element so look for distant galaxies that have strange ratios of certain elements (like oxygen or sulfur) Population III stars might make a special type of extremely bright supernova, so look for distant galaxies that appear bright for a short amount of time and then get dim (transients)
Population III stars: reionization solution? Maybe we're wrong and there's another solution (e.g. more AGN than we predict). Ideas go in and out of vogue in astrophysics and are either refuted or persist and become part of the body of knowledge we think about and teach to ASTR 201 students. One of the science goals of the James Webb Space Telescope (JWST the next Hubble) is to look for evidence of Population III stars. Until then, the jury is still out...
Big Picture Scientists who study the early Universe (since the CMB) are guided by reionization and trying to 'find enough high energy photons' to account for all of the hydrogen that went from neutral to ionized There are a few solutions to the reionization problem that are currently in vogue one of which is the existence of a special nometal Population of stars (Pop III) that are thought to have more mass per star than current populations and therefore produce more UV and X-ray photons The jury is still out on whether Population III stars exist. Some signatures of their existence might be strange abundance patterns in the early Universe or bright supernova events that we might be able to detect with new space telescopes