Starts With A Bang

Ask Ethan: How Big Was The Universe When It Was First Born? (Synopsis)

Image credit: NASA, ESA, R. Windhorst, S. Cohen, and M. Mechtley (ASU), R. O’Connell (UVa), P. McCarthy (Carnegie Obs), N. Hathi (UC Riverside), R. Ryan (UC Davis), & H. Yan (tOSU).

“They say it all started out with a big bang. But, what I wonder is, was it a big bang or did it just seem big because there wasn’t anything else drown it out at the time?” -Karl Pilkington

Looking out at the distant stars, galaxies and radiation in the Universe today, we’ve been able to determine not only what it’s made out of, but how long it’s been since the Big Bang: 13.8 billion years. Put all that information together, and you can also figure out how large the observable part of that Universe is today. From our point of view, it appears to extend for 46.1 billion light years in all directions.

Image credit: Sloan Digital Sky Survey (SDSS), including the current depth of the survey.

So what if you extrapolate backwards, to the very end of inflation and the start of the hot, dense state we identify with the Big Bang, and ask how large that 46.1 billion light year “size” was back then? How big would it be? Depending on the particulars of when inflation came to an end, the answer is somewhere between the size of a soccer ball and the size of a city block, no smaller and no larger.

Image credit: E. Siegel, of the size of the Universe (in light years) vs. the age of the Universe (in years).

Come find out all the gory beautiful details on this week’s Ask Ethan!