From Ethan Siegel: “Is There A Center Of The Universe”
From Ethan Siegel
Feb 11, 2017
The Universe looks roughly the same in all directions, but distant galaxies appear younger and less evolved than nearer ones. Image credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski (Arizona State University, Tempe), R. O’Connell (University of Virginia), P. McCarthy (Carnegie Observatories), N. Hathi (University of California, Riverside), R. Ryan (University of California, Davis), H. Yan (Ohio State University), and A. Koekemoer (Space Telescope Science Institute).
Our Universe began from a Big Bang, but that doesn’t mean we picture it correctly. Most of us think of it as an explosion: where everything started out hot and dense all at once, then expanded and cooled as the different fragments sped away from one another. But as tempting as that picture is, it isn’t correct at all. This led Jasper Evers to ask a very good question:
I am wondering how there isn’t a centre of the universe and how the cosmic background radiation is [equally] far away everywhere we look. It seems to me that when the universe expands … there should be a place where it started expanding.
Let’s think about the physics of an explosion for a moment, and what our Universe would be like if it began from one.
The first stages of the explosion of the Trinity nuclear test, just 16 milliseconds after detonation. The top of the fireball is 200 meters high. Image credit: Berlyn Brixner, from July 16, 1945.
An explosion begins at a point, and expands outwards rapidly. The fastest-moving material moves outwards the most rapidly, and hence spreads out the fastest. The farther away you are from the center of the explosion, the less material will reach you. The energy density goes down as time goes on everywhere, but it goes down faster farther away from the explosion, because the energetic material is more sparse at the outskirts. No matter where you are, you’ll always be able — assuming you’re not destroyed — to reconstruct the center of the explosion.
The large-scale structure of the Universe changes over time, as tiny imperfections grow to form the first stars and galaxies, then merge together to form the large, modern galaxies we see today. https://www.pinterest.com/CosmosUp/galaxies/
But this isn’t the Universe we see. The Universe looks the same at large and short distances: the same densities, the same energies, the same galaxy counts, etc. The objects far away, moving away from us at greater speeds, don’t appear to be the same age as the objects closer to us which move at slower speeds; they appear younger. There aren’t fewer objects at great distances, but more of them. And if we take a look at how everything is moving in the Universe, we find that despite the fact that we can see out for tens of billions of light years, the reconstructed center lands right on us.
From Nature The Laniakea supercluster of galaxies R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède at http://www.nature.com/nature/journal/v513/n7516/full/nature13674.html. Milky Way is the red dot.
Does that mean we, out of all the trillions of galaxies in the Universe, happened to be at the center of the Big Bang? And that the initial ‘bang’ was configured in just such a way — with irregular, inhomogeneous densities, energies, ‘start times’ and a mysterious 2.7 K glow — to conspire so that we’re at the center? What an ungenerous Universe it would be if that were the case: to configure itself in this incredibly unrealistic way at the start.
An explosion in space would have the outermost material move away the fastest, which means it would get less dense, would lose energy the fastest, and would display different properties the farther away you went from the center. It would also need to expand into something, rather than stretching space itself. Our Universe doesn’t support this. Image credit: ESO.
Instead, what General Relativity predicts is not an explosion, but an expansion. A Universe that begins from a hot, dense state has its very fabric expand. There’s a misconception that this would have started from a single point; it isn’t so! Instead, there’s a region that has these properties — filled with matter, energy, etc. — and then the Universe evolves under the laws of gravity.
It has similar properties everywhere, including density, temperature, number of galaxies, etc. If we were to look out, though, what we’d see would be evidence of an evolving Universe. Because the Big Bang happened everywhere at once a finite amount of time ago in a region of space, and that region is all that’s observable to us, when we look out from our vantage point, we’re seeing a region of space that’s not so different from our own position in the past.
Looking back to great cosmic distances is akin to looking back in time. We are 13.8 billion years since the Big Bang where we are, but the Big Bang also occurred everywhere else we can see. The light-travel-time to those galaxies means we’re seeing those distant regions as they were in the past. Image credit: NASA, ESA, and A. Feild (STScI), via http://www.spacetelescope.org/images/heic0805c/.
Galaxies whose light took a billion years to get here appear as they were a billion years ago; galaxies whose light took ten billion years to get here appear as they were ten billion years ago! 13.8 billion years ago, the Universe was dominated by radiation, not matter, and when the Universe first formed neutral atoms, that radiation still persists, having been cooled and redshifted due to the expanding Universe. What we perceive as the Cosmic Microwave Background is not only the leftover glow from the Big Bang, but this radiation is observable from any location in the Universe.
Only a few hundred µK — a few parts in 100,000 — separate the hottest regions from the coldest when we look back at the Cosmic Microwave Background. Image credit: ESA and the Planck Collaboration, via http://crd-legacy.lbl.gov/~borrill/cmb/planck/217poster.html.
There isn’t necessarily a center to the Universe; what we call a “region” of space where the Big Bang occurred could be infinite. If there is a center, it could literally be anywhere and we wouldn’t know; the part of the Universe we can observe is insufficient to reveal that information. We’d need to see an edge, a fundamental anisotropy (where different directions appear different) in temperatures and galaxy counts, and our Universe, on the largest scales, really does look the same everywhere and in all directions.
Artist’s logarithmic scale conception of the observable universe. Image credit: Wikipedia user Pablo Carlos Budassi.
There isn’t a place where the Universe started expanding because of the Big Bang; there’s a time when the Universe began expanding. That’s what the Big Bang is: a condition affecting the entire observable Universe at a specific moment. It’s why looking to greater distances in all directions means looking back in time. It’s why all directions appear to have roughly uniform properties. And it’s why our story of cosmic evolution can be traced back as far as our observatories can see.
Galaxies similar to the Milky Way as they were at earlier times — and greater distances — in the Universe. Image credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University), and the 3D-HST Team.
Perhaps the Universe has a finite shape and a finite size, but if it does, that information is inaccessible to us. The portion of the Universe observable to us is finite, and that information isn’t contained within it. If you think of the Universe as a balloon, a loaf of bread or any other analogy you like, remember that you’re only able to access a tiny part of the actual Universe; what’s observable to us is only a lower limit on what’s out there. It could be finite, it could be infinite, but what we’re sure of is that it’s expanding, it’s getting less dense, and the farther away we look, the farther back in time we’re able to see. As astrophysicist Katie Mack says:
“The Universe is expanding the way your mind is expanding. It’s not expanding into anything; you’re just getting less dense.”
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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan