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:
- 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.
- 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. Observations of the CMB are the bread and butter of our research, so more on that later.
- Light elements (Helium-4, Helium-3, Deuterium, Lithium-7) exist throughout the universe in abundances that are easily explained by nuclear physics operating when the universe was very young (a few minutes old); those abundances 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 I 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!