The Most Successful Cosmological Model
The cosmos is all that is, ever was, or ever will be. The known is finite, the unknown infinite; the cosmic ocean is as vast as inviting. But we have learned most of what we know standing on its shores. Recently, we have waded a little out to sea, enough to dampen our toes or, at most, wet our ankles. But where we stand today is a collective achievement of all those that have come before us. Aryabhata studied the moon, Ptolemy gave us a geocentric model, Copernicus pulled us back on course, Kepler moved the world with elliptic orbits, Galileo looked at the skies, Newton defined gravity, Einstein changed it, and the rest is history. You could argue (and rightfully so) that all of human effort peering into the skies has resulted in what we today may consider the most successful cosmological model: the Λ Cold Dark Matter Model (ΛCDM).
I will try to skip the numbers and mathematics as far as I can. This section only stands to motivate all that follows.
The story told today perhaps does not do justice to the grandness of it all. The general attitude adopted towards science is to rush through history to get on with the actual science of it all. But to do science, one must understand how it came to be. Without it, we are merely children doing as we’re told. The story for the ΛCDM can be sought in the following way: we didn’t believe in Λ, we did believe in CDM, in 1998 something changed, then all of a sudden we did believe in Λ, and so we had ΛCDM. The actual history was, of course, rather more involved — to the point where this oversimplification verges on disingenuous.
We begin on May 20, 1984, in New Jersey. Discovery of the cosmic microwave background (CMB) radiation was something to write home about; it rid every scientist of their justified skepticism towards Big Bang cosmology. From that point on, it was generally accepted that the universe started in a hot, dense state and has been expanding over time. But the model governing this hot dense state was still up for debate.
During the early 1980s, most research focused on pure-baryonic models (models that only included ordinary matter, which you and I have grown to love). When CMB joined the party, however, it brought along a puzzle. It was quickly shown to be smooth. Too smooth, in fact. The early universe was demonstrated to be both isotropic and homogeneous. At the time, scientists were unable to detect any density variations (anisotropies) that could further evolve into galaxies and larger cosmic structures. Though today, these fluctuations are well measured (thanks to WMAP), this was not the case in the late 1900s. This posed a real threat to our then-working understanding of galaxy evolution and formation. But even after discovering anisotropies, we hit a wall.
For a structure to grow, it needed the helping hand of the gravity of some unseen substance. Normal matter did not suffice. This substance had to be moving slowly enough for any real formation physics to take place. It also had to be made out of particles not within the immediate menagerie of the standard model. It was soon realized that this could be resolved if cold dark matter dominated normal baryonic matter. Thus came to be the CDM. CDM is thought to consist of matter other than protons and neutrons (and electrons, although they’re not baryons). It is thought to be cold (in the sense of velocity), therefore excluding neutrinos. It is also held to be disipationless, meaning that it cannot cool through means of radiation. Further to this, cold dark matter particles interact with each other and other particles only through gravity and possibly the weak force. All of this satisfies the constraints required for dark matter to be the hand that helps galaxies form.
At this point, Λ was unmentionable; having garnered a reputation for being Einstein’s biggest mistake, scientists stayed away from it. It did, however, begin to seriously reappear in literature around the early 1990s (see also this). Things radically changed in 1998. Saul Perlmutter, Adam Riess, and Brian Schmidt together broke physics and a Nobel prize followed. Their work showed that the universe’s expansion was accelerating, and not decelerating as previously thought before. This was a big change, one that did not rest well with the previously accepted Standard CDM model. We needed Λ.
So far, we’ve barely spoken of Λ. Λ is the cosmological constant. Qualitatively, it can be thought of as the term responsible to overcome the effect of gravity. Einstein’s cosmological constant was originally necessary to counteract gravity since the Einstein-de Sitter universe considers a static model. This was discarded after Hubble discovered that the universe was actually expanding. To this end, Einstein discarded the cosmological constant. From the 1930s to the 1980s, Einstein’s choice of setting Λ = 0 was generally accepted by the cosmological community. In terms of the physics of it all, Λ = 0 means that the universe’s expansion would slow down and eventually result in contraction, provided there was sufficient density (the Big Crunch). In other words, without a cosmological constant, the universe would expand following some initial momentum of the big bang, but it would decelerate. But with the discovery of an accelerating expansion of the universe, in 1998, we needed to bring back Λ. Today, Λ in some ways is the term responsible for representing dark energy. It allows us to mathematically encapsulate the fact that the universe’s expansion is accelerating.
Coming to the model itself. ΛCDM is a parametrization of Big Bang cosmology. The model posits the existence of three primary components of our universe. First, a cosmological constant responsible for accelerating expansion; second, cold dark matter; third, ordinary matter. Together, these form the very pinnacle of modern cosmological understanding. Fairly straightforward, right?
The reason that it is so widely accepted today as the standard model of cosmology is that it offers a compelling explanation for a large number of cosmic properties: the existence of the CMB, the distribution of galaxies, the observed abundance of lighter elements, and the accelerating expansion of the universe. Another important feature of the model is that it does not (often) violate the cosmological principle. The ΛCDM model has been shown to satisfy the cosmological principle, which is just physics talk for saying that the universe looks the same regardless of where you are. However, recent findings have suggested that violations of the cosmological principle, especially of isotropy, exist. These violations have called the ΛCDM model into question, with some authors suggesting that the cosmological principle is now obsolete (Abdalla et al, Krishnan et al., Heinesen et al.).
With that, I will end this here. I hope it is fairly clear what the most successful model of cosmology looks like and how it came to be. This article was on the lighter sides because I’m working on something bigger so thank you for bearing with me through this! If I have made any errors, please feel free to rip me apart in the comments below. As always, criticism is appreciated. Thank you for reading!
