Cosmology— the beginning
I was originally inspired to study cosmology by a wonderful set of 36 video lectures by Professor Mark Whittle, Cosmology: The History and Nature of Our Universe, 2008. https://www.thegreatcourses.com/courses/cosmology-the-history-and-nature-of-our-universe — It appears that this course material is also freely available online.
What is cosmology?
It’s been roughly 17 years since his course was developed, and though the vast majority of it is still valid, there have been changes, mostly due to more advanced observational capabilities. But new observations from many new tools, such as the James Webb Space Telescope (JWST), have given us new insights into the early and evolving universe. Still, I will likely borrow heavily from material in his course, starting with his definition of cosmology: “the study of the universe as a whole, its origin, its growth, its nature, and its future.”
This is obviously a very broad remit, and it draws heavily upon observational evidence to develop and verify theoretical models. Much of this evidence comes from astronomy and astrophysics, but it also draws from many other fields of study, including (but not limited to) quantum theory, particle physics, nuclear physics, and theoretical physics (especially general relativity); and we will likely find occasions to delve deeper into some of those fields to understand the cosmological consequences.
The birth of cosmology
I want to start with how astronomers establish distances to astronomical objects. This relies on one of science’s most elegant workarounds, something called the cosmic distance ladder. The first rung of this ladder is trigonometric parallax. This works just like the way you estimate distances using your two eyes, which, because they are separated from each other by a few inches, perceive a slight angular displacement of nearby objects. For objects that are farther away, you need a larger separation, called a baseline, than the distance between your eyes, and astronomers get this by measuring displacements between opposite sides of Earth’s orbit, giving a baseline of two AU (an astronomical unit is the radius of Earth’s orbit). This works for the nearest stars within our galaxy.
The next rung on the ladder uses stars called Cepheid variables, whose brightness changes at regular intervals depending on the luminosity (intrinsic brightness) of the star. Given the pulsation period and the observed brightness, the distance to the star can be calculated. This works for nearby galaxies (around 2,000-3,000 large ones) and up to around 65 million lightyears (Mly). This allowed Edwin Hubble to prove in 1924 that Andromeda was a separate galaxy, effectively “enlarging” the known universe overnight.
To measure the distances to the edges of the observable universe, we needed something much brighter than a single star. In the late 20th century, astronomers began using type Ia supernovae. This particular type of supernova occurs in binary star systems where a white dwarf pulls matter from a companion until it reaches a critical mass (the Chandrasekhar limit), at which point it explodes. Because they always explode at roughly the same mass, they always have the same peak luminosity.
In addition to the distance ladder, astronomers can also get some idea of distances by measuring astronomical redshift, which is like the Doppler shift (the change in pitch of a siren as an ambulance passes), but for light, to measure radial velocities of astronomical objects. But redshift is not simply due to Doppler shift. Since the universe is expanding, a more accurate perspective sees the galaxies fixed in space, and it’s the space between us that is expanding, carrying the galaxies along with it. In addition, as the light passes through expanding space, the light’s wavelength is also stretched, increasing the redshift even more.
In 1912, an astronomer named Vesto Slipher was the first to observe the shift of spectral lines of galaxies, making him the discoverer of galactic redshifts, and using this method over the next few years he discovered that other galaxies are moving away from us at high speeds. Although the accuracy of the distance ladder decreases with distance, the accuracy of redshift increases with distance. If you like analogies, you could think of the distance ladder as an odometer, and redshift as a speedometer. And because the speed of light is constant, we’re actually seeing into the past.
Meanwhile, there was progress on the theoretical front:
In 1916, Einstein published his theory of general relativity
In 1922, Alexander Friedmann derived a solution to Einstein’s equations (based on some simplifying assumptions that I’ll discuss later) for an expanding universe
In 1927, Georges Lemaître first linked expansion theory with galaxy observations
And then in 1929, Edwin Hubble published what has come to be known as the Hubble-Lemaître Law (more commonly referred to as Hubble’s Law, but was officially renamed in 2018) with definitive observational data that established that the universe is expanding.
This has had enormous consequences, and these events were later recognized as the theoretical and observational “birth” of cosmology. However, if you ask a physicist when cosmology became a “hard science,” they will likely point to the discovery in 1964 of the Cosmic Microwave Background (CMB), the “afterglow” of the Big Bang, important topics which I will address in future posts.
Einstein’s greatest blunder
It’s interesting to note that in 1917 when Einstein was applying his theory of general relativity to the entire universe, he discovered a problem. His original equations showed that a universe filled with matter could not be stable. Because gravity is an attractive force, all the matter in the universe should eventually pull itself together, causing the entire cosmos to collapse into a single point. The only other mathematical alternative his equations allowed was a universe that was expanding (where the initial outward momentum counteracts gravity).
At the time, the scientific consensus was that the universe was static and eternal. The idea that the universe had a beginning or was changing size felt more like mythology or religion than “”serious” physics. Einstein shared this bias. He was so convinced the universe was unchanging that he felt his equations must be “broken” if they didn’t allow for a stable, unmoving cosmos. To fix this, Einstein added a new term to his field equations: the cosmological constant, denoted by the Greek capital letter lambda, Λ, a sort of “energy of space” that provided a constant outward pressure. He fine-tuned the value of Λ so that its outward push exactly canceled out the inward pull of gravity, resulting in a static universe.
In 1929, when Edwin Hubble proved the universe was actually expanding, and Λ became unnecessary. Einstein reportedly discarded it, calling it his “greatest blunder” because he had missed the chance to predict the expansion of the universe mathematically before it was observed.
However, in 1998, astronomers discovered that the expansion of the universe is accelerating. To explain this, they brought Λ back from the dead. For some years, the cosmological constant has been the leading candidate for dark energy, though rather than just balancing the force of gravity, dark energy now represents a push that is stronger than gravity.
Even more recent and tantalizing results appear to be calling this idea into question, raising the possibility that dark energy evolves over time. If these results hold up, they may even invalidate our current best cosmological model, ΛCDM. That would be exciting, and lead us to an even better understanding of our universe.
Some questions I plan to address in coming posts, not necessarily in this order:
What does it mean for the universe to be expanding?
What is the thermal history of the universe?
What are the major stages of the evolution of the universe?
What is the Big Bang?
What is the CMB, and what have we learned from it?
What is the structure hierarchy of the universe?
How has the structure of the universe changed over time?
What is the Great Attractor?
How has the matter content of the universe changed over time?
What is the difference between the Standard Model of Cosmology and the Lambda Cold Dark Matter (ΛCDM) model?
What is the Hubble “tension”?
What is the evidence for dark matter?
What is the evidence for dark energy?