365daysDate: June 17, 2009

Title: The Big Bang!


Podcaster: Josh Schroeder from Columbia Astronomy

Organization: Columbia University Astronomy

Description: The universe used to be a very different place, billions of years ago. The temperature and density was so high that galaxies, stars, planets, and even atoms could not exist. Luckily, for us, the universe expanded and cooled off. Astronomers and physicists have been able to describe and model the processes by which the universe transitioned from that hot, dense, alien environment to the conditions we observe today. The space, time, matter, and energy we take for granted in our everyday lives all owe their characteristics to events that took place a finite time in the past we call the “Big Bang”.

Bio: Joshua Schroeder is a graduate student at Columbia University where he studies astronomy. His research interests include cosmology, the epoch of reionization, binary stars, and spectroscopy. Schroeder has a M.S. degree in astrophysical and planetary sciences from the University of Colorado, Boulder, and a Bachelor’s degree in astrophysical sciences from Princeton University.

Today’s sponsor: This episode of 365 Days of Astronomy is sponsored by AAS.


Hello everyone, and welcome to Columbia Mondays’ special Wednesday
edition! My name is Joshua Schroeder, and I’m a second-year graduate
student at Columbia University in the City of New York. Today the topic of
our podcast is going to be….

The Big Bang

Ask your average, moderately educated astronomy enthusiast the question How did the universe begin? and you will get back the response, The Big Bang! But what does this answer mean? What is the Big Bang? Some people describe it as the explosion that started the universe‚ but is it really an explosion? Is it even, really, a beginning? In order to answer these questions, let’s explore the evidence and the theories about the way our universe was some 13.7 billion years ago, a time popularly known as The Big Bang.

Looking out into the universe, we see hundreds of billions of galaxies each containing hundreds of billions of stars. Regardless of where we see them, these galaxies all follow the trend that as they get farther away from us they recede at speeds that increase with their distance. This is because the space between galaxies is expanding. An analogy astronomers use is to compare our view of the universe to the view a raisin would have in a raisin bread that is rising in the oven. Each raisin sees all the other raisins in the bread moving away from it at speeds that increases with the distance. This is because the dough between the raisins is expanding.

The expansion of space in our universe is predicted by the equations of general relativity which explain how space and time are affected by geometry and curvature. General relativity predicts a special feature of the universe called the cosmic scale factor that changes through time.

As time increases, so does this cosmic scale factor with the effect that the distances between points in space increase as time goes forward. It is this increase in the cosmic scale factor that causes the distance between objects to grow larger as time progresses and causes the recessional speeds of galaxies to increase with increasing distance.

Since the scale factor is increasing forward in time, then it must be decreasing backward in time. Due to the finite speed of light, looking out into the universe is essentially the same as looking backwards in time. An object that is one million light years away is observed by us as it was one million years ago. When we look at the distant universe, we see that the galaxies were closer together, that the universe was hotter, and that conditions were much different due to the smaller scale factor in the earlier universe.

Extrapolating this idea to its natural conclusion means that at some time in the finite past, the cosmic scale factor was so small that distances between all the galaxies approached zero. That means that all the space we observe must have been at the same place at some point in the past. By measuring the rate that the cosmic scale factor is changing, scientists can measure how long ago in the past this instant must have been. This moment occurred some 13.7 billion years ago and is the moment that we usually associate with the term The Big Bang. A shorthand way of describing this event is to say that the universe started when the cosmic scale factor equaled zero and cosmic distances in the universe have been increasing to this day.

You may be wondering, what characterizes this moment in time when the cosmic scale factor was zero? The answer is a bit mysterious. In fact, it’s not known yet whether the scale factor was really every exactly zero (which corresponds to a physically paradoxical singularity) or whether it was just really, really small. It may be that there was no zero-point at all and indeed there exist theories and speculations which do not require an initial singularity, though we do not yet have enough information to determine which of these ideas is correct. Even though it is possible that the universe existed before the point we describe as The Big Bang, astronomers still say that the Age of the Universe is 13.7 billion years old and the universe started in the Big Bang. This is because, from what we understand of the physics of the situation, the Big Bang is the moment where our universe first started to behave with the characteristics that we observe our universe to have today: an expanding scale factor with a finite amount of matter, energy, three dimensions of space and one dimension of time.

Even if the scale factor was never exactly zero, we do know from observations of the cosmic microwave background (the so-called Echo of the Big Bang) that when the universe had a small cosmic scale factor it also had a very high matter and energy density meaning that the universe was very hot and matter was squished together in ways that we only can only begin to probe in the largest particle accelerators. The temperatures in the early universe were so high, in fact, that matter could not take the form of molecules, atoms, or even subatomic particles. In the earliest universe, theoretical physicists predict the conditions in the universe permitted matter and energy to freely switch forms according to Einstein’s most famous equation, E=mc2. Indeed, distinctions like matter, energy, particle, and radiation were essentially meaningless.

The cosmic scale factor changed rapidly at first in a period cosmologists call inflation. As the universe continued to expand and cool, processes analogous to the freezing of water happened in what cosmologists call freeze out and what chemists call fractional distillation. The almost equal amounts of matter and anti-matter that was found in the earliest universe annihilated into photons leaving behind a tiny excess of matter over anti-matter that persist to this day. The heavier subatomic particles froze out first, followed by the lighter ones, with atomic nuclei forming in the first three minutes through nuclear fusion processes.

Atoms themselves weren’t able to form until after approximately three hundred thousand years. Within a billion years the first generation of stars formed until the assemblages of galaxies with the complex structures we see appeared. All the atoms that make up the gas, stars, planets, living, and non-living matter you seen around you every day owe their existence to the processes that occurred in these early moments of the Big Bang.

Though the Big Bang as a theory has been fabulously successful in explaining many of the features of our universe, tremendous mysteries lay yet to be explored. A significant fraction of the matter in our universe is in a form called dark matter which has only been indirectly detected as of yet by its gravitational interactions. Additionally, within the last decade astronomers have measured a peculiar form of dark energy that seems to be causing the cosmic scale factor to increase more quickly now than it did in the past. This form of energy is so utterly strange that the most basic models we have of it contain physical paradoxes that confound the theorists trying to account for it. We still have a lot yet to learn about what these features of our universe are and how they affect the Big Bang, and the excitement of our discovery about the universe continues.

This has been a podcast of Columbia University here in the City of New York. For more information about public events of Columbia Astronomy visit Our next Columbia Monday podcast will be by Sarah Tuttle on Monday, July 27th on the topic of Astronomical Ballooning Or, What Goes Up Must Come Down. Have a great day and keep listening.

End of podcast:

365 Days of Astronomy
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