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Starts With A Bang

This Is Why ‘Physical Cosmology’ Was Long Overdue For The 2019 Nobel Prize

In the mid-20th century, ‘physical cosmology’ was considered an oxymoronic joke. Today, it’s Nobel-winning science.

Imagine you wanted to know everything you could about the Universe. You’d want to find the answers to questions of all types, such as:

  • What is the Universe made of?
  • How big is the entire Universe?
  • How long has the Universe existed?
  • What was the Universe like in its earliest stages?
  • What types of structures exist, and when/how did they form?
  • How many galaxies do we have?
  • How did the Universe grow up to be the way it is today?
  • And what is its ultimate fate in the far future?

It’s a daunting task to ponder. And yet, there’s a method of thinking that can give you the answer to all of these questions and many more: the method of applying physical cosmology. Earlier this October, the 2019 Nobel Prize in Physics was awarded jointly to Michel Mayor and Didier Queloz (for exoplanet discoveries) and Jim Peebles (for physical cosmology). While exoplanets are easy to understand — planets outside of our own Solar System — physical cosmology needs an explanation. Here’s the amazing story.

Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and evolution, and continues to do so. Although we have a large amount of evidence for dark matter, it doesn’t really make its presence known until many years have passed since the Big Bang, which means that dark matter may have been created at that time or earlier, with many scenarios remaining viable. (NASA / CXC / M.WEISS)

If you want to understand any object or phenomenon in the Universe, there are many different lines of approach. You can observe it in all the different ways you can imagine. This includes detecting its light in different wavelength bands; looking for spectroscopic signatures of various elements; measuring observable properties that are linked to intrinsic properties; measuring its redshift; searching for particles or gravitational waves emitted from it, etc.

However, regardless of what you’re measuring, one fact remains true about any and all structures and objects in existence: they all formed naturally in a Universe governed by the same laws and made up of the same components everywhere. Somehow, natural, physical processes occurred to take the Universe from how it was at early times and transformed it into the objects and phenomena we observe today. The key for physical cosmology is to figure out how.

An illustration of gravitational lensing showcases how background galaxies — or any light path — is distorted by the presence of an intervening mass, but it also shows how space itself is bent and distorted by the presence of the foreground mass itself. Before Einstein put forth his theory of General Relativity, he understood that this bending must occur, even though many remained skeptical until (and even after) the solar eclipse of 1919 confirmed his predictions. There is a significant difference between Einstein’s and Newton’s predictions for the amount of bending that should occur, due to the fact that space and time are both affected by mass in General Relativity. (NASA/ESA)

Imagine the Universe as a scientist might have imagined it a century ago: shortly after the arrival and first confirmation of General Relativity. Before any other observations are considered — and scientists were still (at the time) debating whether the Milky Way galaxy was the entirety of the Universe or whether those fuzzy spirals and ellipticals were actually their own galaxies far beyond our own (spoiler: they are) — the seeds of modern physical cosmology had already sprouted.

In physical cosmology, what you do is you start with:

  • the known laws of physics,
  • the relevant physical ingredients for the system you’re considering,
  • the initial conditions of your physical system that your Universe begins with,
  • and an accurate model for the interactions among the ingredients (including the background of spacetime).

Once you have all of that, then you do the calculations to derive what you expect to exist within our Universe.

Large scale projection through the sophisticated cosmic structure formation simulation: Illustris. This represents the most massive cluster in the simulated Universe, with a scale that’s 15 Mpc/h (about 70 million light-years) deep. The visualization shows dark matter density (left) transitioning to gas density (right). The large-scale structure of the Universe cannot be explained without dark matter, though many modified gravity attempts exist. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

When your observations come in, you compare them with your theoretical expectations. Where observational and theoretical cosmology meet is where we, at long last, can scientifically determine what does and doesn’t accurately describe our Universe.

In the earliest days of General Relativity, the science of physical cosmology was in its most rudimentary stages. But even a primitive beginning is still a beginning, and what scientists began to derive were classes of exact solutions in General Relativity. In other word, you can make a simplifying assumption about what the properties of the Universe are, and you can write down the equations that describe a Universe that obeys those conditions under our best laws of gravity. By the end of the 1920s, we had solutions for:

  • a Universe that was empty (Milne Universe),
  • a Universe that contained one point mass (a non-rotating, Schwarzschild black hole),
  • a Universe that contained a cosmological constant (de Sitter space),
  • a Universe that was filled with normal matter alone (an Einstein-de Sitter Universe),
  • and, generally, a Universe that could be filled with anything, so long as it was isotropic (the same in all directions) and homogeneous (the same at all locations in space).
A photo of me at the American Astronomical Society’s hyperwall in 2017, along with the first Friedmann equation at right. The first Friedmann equation details the Hubble expansion rate squared on the left hand side, which governs the evolution of spacetime. The right side includes all the different forms of matter and energy, along with spatial curvature, which determines how the Universe evolves in the future. This has been called the most important equation in all of cosmology, and was derived by Friedmann in essentially its modern form back in 1922. (PERIMETER INSTITUTE / HARLEY THRONSON)

That last option gives rise to a set of equations — the Friedmann equations — with a number of fascinating consequences. First off, they predict an expanding or contracting Universe; a static one is unstable. Second, they teach you how the different possible types of energy density (e.g., normal matter, dark matter, neutrinos, radiation, dark energy, domain walls, cosmic strings, spatial curvature, and anything else you can cook up) will not only evolve with time, but they will describe how the expansion rate changes over our cosmic history as well.

By measuring not only how quickly the Universe is expanding today, but by measuring (through a variety of observational techniques) how the expansion rate has changed over time, we can begin to extract some detailed information concerning what makes up our Universe.

Measuring back in time and distance (to the left of “today”) can inform how the Universe will evolve and accelerate/decelerate far into the future. We can learn that acceleration turned on about 7.8 billion years ago with the current data, but also learn that the models of the Universe without dark energy have either Hubble constants that are too low or ages that are too young to match with observations. If dark energy evolves with time, either strengthening or weakening, we will have to revise our present picture. This relationship enables us to determine what’s in the Universe by measuring its expansion history. (SAUL PERLMUTTER OF BERKELEY)

So yes, by measuring how quickly the Universe is expanding and how that expansion rate has changed over time, we can infer what the Universe’s density is, what the various components are that it’s made out of, and even — if we can determine those parameters precisely enough — what the ultimate fate of the Universe would have to be. This is the most basic example of physical cosmology: applying the laws of physics to the entirety of the expanding Universe.

Of course, this approximation is going to be good for some thing and not others. On average, it should be able to tell you how the Universe is expanding on the largest scales of all. But for all the other consequences, we have to consider some of the physical properties and particle interactions that must occur, but that we purposefully omitted earlier.

Our entire cosmic history is theoretically well-understood in terms of the frameworks and rules that govern it. It’s only by observationally confirming and revealing various stages in our Universe’s past that must have occurred, like when the first elements formed, when atoms became neutral, when the first stars and galaxies formed, and how the Universe expanded over time, that we can truly come to understand what makes up our Universe and how it expands and gravitates in a quantitative fashion. The relic signatures imprinted on our Universe from an inflationary state before the hot Big Bang give us a unique way to test our cosmic history, subject to the same fundamental limitations that all frameworks possess. (NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

One thing we can do is to consider a Universe that includes normal matter (including protons, neutrons and electrons) and radiation (such as photons), as well as the interactions that govern such particles. When the Universe started off, it was mostly uniform, but also contained this matter and this radiation. It was also hotter, as an expanding Universe stretches the wavelengths of photons within it, making them less energetic over time.

If we extrapolate back to the past, we can calculate that there was an early time (and a high enough temperature) in the Universe’s past where the formation of neutral atoms would have been impossible, as photons would have blasted them apart into an ionized state. In order to calculate when that occurred, you have to calculate all the atomic physics necessary to learn when the Universe’s atoms become stably neutral, and how that impacts what we’ll see today as the leftover radiation from the Big Bang: the cosmic microwave background (CMB).

At early times (left), photons scatter off of electrons and are high-enough in energy to knock any atoms back into an ionized state. Once the Universe cools enough, and is devoid of such high-energy photons (right), they cannot interact with the neutral atoms, and instead simply free-stream, since they have the wrong wavelength to excite these atoms to a higher energy level. (E. SIEGEL / BEYOND THE GALAXY)

At even earlier times, you can perform the analogous calculation for atomic nuclei, and see where collisions blast composite nuclei apart into protons and neutrons versus where they’re no longer energetic enough to do so. When you then go and measure the abundances of these light elements (by probing gas clouds that have never formed stars), you should see a specific ratio of elements like hydrogen, deuterium, helium-3, helium-4, and lithium-7.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

If you go even earlier and understand that the early Universe must have had high enough energies to spontaneously produce matter-antimatter pairs (and understand how fermions like neutrinos obey different rules than bosons like photons), you can calculate the ratio of the cosmic neutrino background’s energy to the CMB’s individual photon energy, since when the electron-positron pairs from the early Universe annihilate away, they only become photons, never neutrinos. The calculation tells us the neutrino temperature is (4/11)⅓ times the CMB temperature; since the latter is 2.725 K, the former must have a temperature equivalent of 1.95 K.

There are peaks and valleys that appear, as a function of angular scale (x-axis), in various temperature and polarization spectra in the cosmic microwave background. This particular graph, shown here, is extremely sensitive to the number of neutrinos present in the early Universe, and correspond to the standard Big Bang picture of three light neutrino species. If we accept that there are three species as a given, we can extrapolate the temperature-equivalent energy inherent to the cosmic neutrino background as opposed to the CMB, and find it to be ~71%, remarkably consistent with the theoretical prediction of (4/11)^(1/3). (BRENT FOLLIN, LLOYD KNOX, MARIUS MILLEA, AND ZHEN PAN (2015) PHYS. REV. LETT. 115, 091301)

Physical cosmology also tells you what sort of structures you expect to find in the Universe. You can start with an almost-homogeneous Universe, but one that has density (and/or temperature) imperfections in it, model the interactions between particles and radiation and include gravitation, and see how these imperfections evolve.

You’ll find that the imperfections evolve according to different behaviors depending on how much normal matter vs. dark matter exists in your Universe, and will leave a specific imprint in the CMB. You’ll find that the overdense regions grow at a quantifiable rate until they reach a critical density, then undergo runaway collapse to form stars, galaxies, and other cosmic structure. The early stars reionize the Universe; the larger-scale structure forms today’s massive cosmic web.

Both simulations (red) and galaxy surveys (blue/purple) display the same large-scale clustering patterns as one another, even when you look at the mathematical details. The Universe, particularly on smaller scales, is not perfectly homogeneous, but on large scales the homogeneity and isotropy is a good assumption to better than 99.99% accuracy. The specific details in the growth of the cosmic web have enormous implications for physical cosmology. (GERARD LEMSON AND THE VIRGO CONSORTIUM)

It is a spectacular fact of modern science that the predictions of theoretical cosmology have been verified and validated by ever-improving observations and measurements. Even more remarkably, when we examine the full suite of the cosmic data humanity has ever collected, one single picture accurately describes every observation together: a 13.8 billion year old Universe that began with the end of cosmic inflation, resulting in a Big Bang, where the Universe is comprised of 68% dark energy, 27% dark matter, 4.9% normal matter, 0.1% neutrinos, and a tiny bit of radiation with no spatial curvature at all.

Put those ingredients into your theoretical Universe with the right laws of physics and enough computational power, and you’ll obtain the vast, rich, expanding and evolving Universe we have today. What was initially an endeavor of just a handful of people has now become the modern precision science of cosmology. In the middle of the 20th century, legendary physics curmudgeon Lev Landau famously said, “Cosmologists are often in error but seldom in doubt.” With the 2019 Nobel Prize in Physics going to Jim Peebles, perhaps the world will recognize it’s long past time to retire Landau’s quote. We may live in a dark Universe, but the science of physical cosmology has shed a light on it like nothing else.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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