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RACHEL BEAN: The 20th century witnessed the major observational and theoretical developments that revolutionized our understanding of the universe and instigated what we today call modern cosmology. Two of the major protagonists are Edwin Hubble, for his observational discoveries, and Albert Einstein, for the theoretical framework in which to understand them.
In the 1920s, Hubble established that fuzzy patches on the sky were, in fact, distant galaxies beyond our own. Measuring distances is one of the most difficult tasks in astronomy, since unfortunately we can't just send a student out with a tape measure. Hubble used a technique discovered by one of the remarkable women in science at the last century, Henrietta Swan Leavitt, to measure distances to galaxies. Leavitt was payed $0.25 an hour to work as a human computer at Harvard University Observatory, poring over photographic plate images of stars and cataloging the variation of their brightness and position over time. She found that for a certain type of variable star, called Cepheid variables, the more luminous stars have a longer period of variability.
Therefore, comparing the apparent brightness of Cepheid variables in distant galaxies to their true luminosity, inferred from the variability period, Hubble was able to infer the distance to the galaxies. He found that they were a billion times more distant than the outer solar system. He also measured how fast they were moving with respect to us by using the Doppler shift of light emitted from the galaxies. Galaxies moving towards us have the wavelength of light compressed, so that it looks more blue. Those moving away have the wavelength expanded, so that it's shifted to the red.
Hubble found that all the galaxies he observed were red-shifted, i.e., they're moving away from us. By plotting a graph of distance versus recession speed, Hubble found the two were proportional to each other. Hubble's law is the formula encoding this relationship between the distance and speed. Hubble's law has been now shown to hold to almost the largest measurable distances in the universe. The figure shows a recent graph of Hubble's law, out to distances 1000 times greater than those measured by Hubble.
How do we interpret Hubble's law? If we think of the motion of galaxies around us, one might think Hubble's law suggests that we are at the center of the universe. However, if we take another observer's perspective in another galaxy, Hubble's observations would also suggest the same egomaniacal implication for them. Instead, we take a more modest perspective, invoking the so-called cosmological principle, that on cosmic scales there is no preferred observer. The universe looks the same in every place and has no preferred direction. In this context, Hubble's observations suggest that there is no center to the universe, and every observer must be seeing the same expansion of space. It is as if we are all on a three-dimensional surface of a four-dimensional balloon, as the balloon is being blown up.
Before Hubble's discovery, Einstein developed his theory of general relativity. General relativity is a general law of gravity, able to describe its properties on all scales, even in regions where gravity is strong, for example around a black hole and over cosmic distances. General relativity postulates that there is a relationship between the presence of matter and the curvature of space. It tells us how space is distorted by matter, and in turn, how matter responds to that distortion of space.
General relativity, or GR for short, reduces to Newtonian gravity when gravity is weak and over short distances. GR tells us that matter tells space how to bend or expand. For example, space is distorted by the presence of Earth's mass. Similarly, space tells matter how to move, so that a satellite placed in Earth's vicinity will move towards the Earth, and you can think about this as it moving with the curvature of space. Finally, on a cosmic level, GR tells us that the presence of matter makes the universe change size, either expand or contract.
Theorists came up with two competing hypotheses for the universe's evolution, both in agreement with Hubble's law and Einstein's general relativity. One was the steady state model, proposed by Fred Hoyle, Hermann Bondi, and Cornell professor Thomas Gold, in 1948. In this picture, the universe will always look the same and had no beginning. They invoked a perfect cosmological principle in which, as well as the universe looking the same at every place, it also looks the same at every point in time. As the universe expands, its density remains constant by new matter being created all the time from nothing.
An alternative theory was proposed by George Lemaitre in 1927, and then advocated by George Gamow in 1948. In this model, the universe is expanding and is always changing. Matter becomes less dense and cooler as it is diluted by the expansion. Going back in history, therefore the universe had a hot and incredibly small beginning, the so-called hot Big Bang. At the time of Hubble's observations, there was no observational test to distinguish between the steady state and Big Bang hypotheses. It was to be roughly another 35 years until the theories were able to be confronted by data.
In 1963, Penzias and Wilson were working at Bell Labs in New Jersey, making ultrasensitive measurements of radio emissions in the Milky Way. What they found was an unexpected emission of photons coming from all directions, seemingly from outside the galaxy. The photons all had the same energy. We usually talk about photon temperature, which is just proportional to energy, and they all have the same temperature of 2.7 Kelvin. That's about minus 450 Fahrenheit. I.e., they were extremely cold, low-energy, microwave photons.
So what was the cause of this emission? The first thought was that, rather than being something extragalactic, maybe the noise was a systematic error caused by a problem in the telescope. One possibility was that it was caused by the guano from pigeons living in the telescope. Being physicists, Penzias and Wilson called the pigeon droppings a white dielectric material. However, it was found that even after the pigeons were permanently relocated, the emissions still persisted.
It was then realized that the emission was actually primordial radiation, a signature of the early universe that theorists in both Russia and the US had predicted. If the Big Bang is true, then the early universe was much hotter and denser than it is today. It was hot enough that the electrons had enough energy to escape from atoms, and the universe was a fully ionized plasma of charged particles. Photons interact with charged particles very readily, so they are constantly scattered, and as a result, the early universe was completely opaque.
As the universe expands, it cools, and at 4,000 years after the Big Bang, it cools enough, to a temperature of 3,000 Kelvin, the electrons no longer have enough energy to escape from atoms, and neutral hydrogen is formed. At this point, the photons no longer get scattered, and the universe from then on is transparent for the photons. As the universe expands, the photons cool, and today the same photons have a temperature of 2.7 Kelvin, rather than their original 3,000 Kelvin, and become the cosmic microwave background, abbreviated to CMB, seen by Penzias and Wilson.
The CMB is only explained in a theory in which the universe and the photon temperature evolve over time. The steady state model therefore was ruled out by the CMB. You can see the CMB for yourself by disconnecting your cable box. About 1% of the hiss on the TV screen is this primordial radiation, the earliest observable fossil of the universe. You can learn more about the CMB and the other vital information it gives us about the universe in the references associated with this talk.
Cosmology uses observations of cosmic structures, like stars and galaxies, to understand the origin, evolution, and ultimate fate of the universe. Join Rachel Bean as she examines our current perception and evolving ideas of the universe.
This video is part 1 of 6 in The Puzzling Life of the Universe series.
