Spergel journeys through time at Bennett-McWilliams lecture

What if you could go back in time? What if you could go so far into the past that you could see the beginning of the universe?

David Spergel, chair of the department of astrophysical sciences at Princeton University, took his audience back in time at the second Bennett-McWilliams lecture in Rashid Auditorium last Thursday with his creatively named talk, “Taking the Universe’s Baby Picture.”

The truth is, you are traveling into the past each time you look up at the night sky. The stars are so far away that by the time the light they are emitting — which travels very fast, but still at a finite speed — reaches your eyes, you are seeing the stars not as they exist in the sky at that moment, but how they were when that light first left the star.

“If you were on a planet 10 light-years away and were looking at Earth, you would see all of us as we were 10 years ago — and I had more hair,” Spergel joked. It makes sense, then, that the farther away an object you are looking at is, the older a version of it you will see.

It is this concept that allows researchers to use the cosmic microwave background (CMB) to study the universe’s history. The CMB is essentially leftover heat from the Big Bang. When the universe was young, it was very hot and very dense, and protons and electrons moved around freely in plasma.

However, as the universe expanded and cooled, protons and electrons began to combine and photons — that is, light — began to decouple from the matter. It is this light, which has moved freely from then to now, which gives us a window into the universe’s initial conditions.

“The microwave background is a wonderful thing to study if you want to understand the evolution of the universe’s history,” said Spergel. “Because it really is, I’ll argue, the universe’s baby picture.”

So what exactly does a map of the CMB look like? You might be familiar with the famous CMB pictures, which look like an oval on which various patches of different colors are dispersed, that have been published by surveys such as Planck and the Wilkinson Microwave Anisotropy Probe (WMAP). These different colors correspond to the slightly different temperatures across the universe.

“What we want to do is look at this pattern and study the ‘lumpiness’ as a function of scale. What is the characteristic scale of the hot and cold spots on this map?” Spergel explained.

To illustrate his point, Spergel showed the baby pictures of his own children — “Three completely randomly selected babies,” he assured, as the crowd chuckled — followed by a more recent picture of them.

“Given the initial conditions, you can extrapolate forward to today,” Spergel explained. “Now, this will not work with babies … but the universe turns out to be more simple.”

Using computer simulations of the initial conditions of our universe and evolving them forward in time, we can compare our predictions of the present universe’s “lumpiness” to actual observations, and it turns out that they match remarkably well.

While the first half of Spergel’s talk began with the universe’s initial conditions and went forward in time to the present day, the second portion of his talk was about how observations made in the present can take people back in time to the universe’s first moments, a discussion which revolved around the theory of inflation.

“The way that I think about the motivation for inflation comes from having taught for a number of years,” Spergel said. He explained the theory with an example: If you gave an exam to a classroom full of students and every student handed back their exams with the same answers, one of two things must have happened — either there was communication between the students during the exam, or the students shared the same information beforehand.

Similarly, when we look at the many different regions of the CMB, all of the patches are the same temperature, with only minuscule fluctuations between them. However, according to the standard Big Bang Theory, these regions of the sky did not have time to communicate to each other, suggesting that they shared a common initial condition. The theory of inflation states that these regions once existed at a single point, and that the universe underwent a rapid, exponential expansion to become the universe we know today.

During this inflation, tiny ripples in the fabric of space-time were blown up, resulting in the gravitational waves that the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope very recently detected, causing a similar ripple of waves to propagate throughout the physics community.

“My reaction to this — and this might be me being a bit of a cynic — is that important results deserve careful scrutiny,” Spergel cautioned. He worried that since the CMB is composed of both cosmological emissions and galactic emissions, the signal that was detected by BICEP2 could include contributions from distant galaxies that were not accounted for.

In the next two years, fortunately, seven different experiments will be independently working to confirm the discovery.

Between the first Bennett-McWilliams lecture — which occurred in December and featured Alan Guth, the theorist behind inflation — and Spergel’s lecture last week, gravitational waves have already been detected. Spergel’s lecture took the audience on an adventure through the past, but who knows what greater discoveries the future has in store for us?