Cosmology is the study of the universe at large: its thermal and dynamical history, the origin and nature of its contents and structures on the largest scales. Our physical model of the universe, the “Hot Big Bang”, is motivated by fundamental physics and rooted in observational fact. Given the scale of the questions we ask in cosmology, it’s amazing that we really know so much!
Our acceptance of the Hot Big Bang model is driven by a few fundamental observations:
Awesome fact #1: The universe is expanding.Observations of distant galaxies show that the farther away a galaxy is, the faster it appears to be receding from us. This fact leads to the Hubble Law,
v = H * d
where v is the apparent velocity found via the galaxy’s “doppler shift”, d is the distance to a given galaxy, and H is the “Hubble Constant” (roughly 70 km/second/Megaparsec). Assuming that the “cosmological principle” is correct, ie that other observers elsewhere in the universe will see the same Hubble Law, we are driven to a model where those apparent velocities are not due to motions through space, but rather the expansion of space itself. We describe that expansion by a scaling factor a(t) that we set to be equal to one now, which increases as the universe expands.This leads us to think about early times when the expansion factor a(t) was much smaller, and therefore the universe was much denser and hotter.
From Neta Bahcall, Hubble’s Law and the expanding universe, Proceedings of the National Academy of Sciences of the United States of America, V 112, 2015
Awesome fact #2: The Cosmic Microwave Background exists!
Predicted and then discovered in 1965, the CMB is a space-filling thermal sea of photons with a temperature of 2.7K. Measurements of its brightness as a function of photon frequency fit the Planck blackbody formula exquisitely well; this tells us that matter and radiation were once in thermal equilibrium in the early universe. Working through the atomic and thermal physics, we find specifically that CMB photons ionized the normal matter (mostly hydrogen) in the early universe, just 380,000 years after the Big Bang. Observations of the CMB are the bread and butter of our research, so more on that later.
Predicted and then discovered in 1965, the CMB is a space-filling thermal sea of photons with a temperature of 2.7K. Measurements of its brightness as a function of photon frequency fit the Planck blackbody formula exquisitely well; this tells us that matter and radiation were once in thermal equilibrium in the early universe. Working through the atomic and thermal physics, we find specifically that CMB photons ionized the normal matter (mostly hydrogen) in the early universe, just 380,000 years after the Big Bang. Observations of the CMB are the bread and butter of our research, so more on that later.
COBE-FIRAS spectrum of the CMB
Awesome fact #3: Light elements (Helium-4, Helium-3, Deuterium, Lithium-7) exist throughout the universe! Their abundances are easily explained by nuclear physics operating when the universe was very young (a few minutes old), but cannot be explained by any reasonable physical models of stellar burning throughout the history of the universe, but are a natural outcome of the Hot Big Bang.
Other observations have led us to a specific version of the Hot Big Bang model that many of us like to refer to as the “standard cosmological model”, which we also call “Lambda Cold Dark Matter Cosmology”, or LCDM for short. Some of the most fundamental and amazing facts that have led us to this model include:
The “normal matter” that we are made of (nuclei, electrons, etc) only accounts for about 5% of the energy density of the universe. This estimate is supported by measurements of brightness variations in the CMB from one place to another across the sky, and measurements of the abundance of light elements cooked up in the first few minutes of the Big Bang (mentioned above).
About 30% of the energy density of the universe is in the form of “dark matter”; the name conveys two of the features of this stuff… it is “dark” (doesn’t shine, so we can’t see it with our telescopes), and it clumps gravitationally just like normal matter does. The formation of large scale structures (galaxies, clusters of galaxies) and dynamics of large objects (galaxies, clusters of galaxies) cannot be explained without invoking the presence of some form of dark matter (unless General Relativity is wrong). The fact that only 5% of the energy density of the universe is “normal matter” means that the remaining 30% required by these observations must be “abnormal”… not made of the atoms, etc, that we’re familar with. Cosmologists call it “non-baryonic dark matter”… and physicists (including a group here at Case) are working hard to see it in the lab.
About 65% of the energy density of the universe is in the form of “dark energy”. This is different than dark matter in two important ways: it doesn’t clump to help form clusters of galaxies, and instead of slowing the expansion of the universe, it causes an increase in the expansion rate as time progresses. The first solid evidence for the existence of dark energy came from measurements of the brightness of distant supernovae, in the mid to late 1990’s. Shortly thereafter, a combination of measurements of the CMB anisotropies, the expansion rate (H), and/or the energy density of matter from Large Scale Structure led to the same conclusion. Currently there’s lots of evidence that dark energy exists and dominates the energy density of the universe, but there’s virtually no understanding of the actual physics of it!
CMB Anisotropies
We've learned, and are continuing to learn, a huge amount about physics and cosmology by mapping the very small variations in the CMB's brightness across the sky. These variations are called "anisotropies", which means "different when you look in different directions". The best all-sky maps of these variation in brightness were made by the Planck satellite, which made the beautiful map shown below; the red and blue areas there indicate real density variations in the early universe's plasma, captured in a "snapshot" when the universe was only 380,000 years old.
From maps like these, including maps of the polarization of the CMB, we've learned an enormous amount about the contents of the universe (Dark Matter, Dark Energy, as well as the regular stuff out there), and its history (how fast it's expanding, for example).
Our work with the South Pole Telescope and CMB-S4 continues that legacy, making better measurements of the CMB that probe new science and new discoveries.