Is Space Curved? Can We See The Milky Way In The Past?

Is it possible that space-time is curved in such a way that one (or many) of the galaxies we see in telescopes is actually our own Milky Way a few billion years earlier?
This infrared view reveals galaxies far, far away that existed long, long ago. Taken by the Near Infrared Camera and Multi-Object Spectrometer aboard the NASA/ESA Hubble Space Telescope, the image is part of the Hubble Ultra Deep Field survey, the deepest portrait ever taken of the universe. Image credit:  NASA ,  ESA  and R. Thompson (Univ. Arizona)

This infrared view reveals galaxies far, far away that existed long, long ago. Taken by the Near Infrared Camera and Multi-Object Spectrometer aboard the NASA/ESA Hubble Space Telescope, the image is part of the Hubble Ultra Deep Field survey, the deepest portrait ever taken of the universe. Image credit: NASAESA and R. Thompson (Univ. Arizona)

Originally posted on Forbes!

It is mathematically possible for a universe to be shaped this way, but not our Universe. Our Universe is as close to flat as we can measure right now, though it’s only possible for it to be very slightly curved, considering the wiggle room we have remaining on our measurements.

The universe that you describe could be round, donut-shaped or cylindrical; some shape where at least in one direction, it connects back to itself. These aren’t your only options for a universe - you could also invent a saddle shaped or other, more exotic shape to place your universe in.

For now, let’s roll with a cylindrical universe. And let’s put a star somewhere on the surface. If the light from this star is going along the length of the cylinder, all it can ever do is go out, because the surface is flat in that direction; there’s no curve or loop. This flat, uncurved behavior is how we believe our Universe behaves in every direction. Light in our Universe departs its star, and travels in a straight line forever (as far as we can tell) unless it is intercepted by another astrophysical object, another star, planet, or telescope detector.

However, the light that leaves our star in the cylindrical universe has one other option. The light that goes in the other direction - around the curve of the cylinder - will also travel in a straight path. But this path loops back on itself, and if the light doesn’t hit anything else, after it has completed its tour of the cylinder’s circumference, it will arrive back where it began, on the other side of the star, delayed by the length of time it took to do its loop.

The three possible geometries of space. At the top is a sphere, followed by a saddle-shaped universe, and then flat. Each geometry will affect the path of light traveling through it. Image credit: NASA/WMAP Science Team

The three possible geometries of space. At the top is a sphere, followed by a saddle-shaped universe, and then flat. Each geometry will affect the path of light traveling through it. Image credit: NASA/WMAP Science Team

 

What happens if you make your universe spherical? It’s a very similar thing, except now every path that light can take will loop back onto itself, given enough time. There’s another curious thing about the light this time, though, which is that the beams of light, even though they’re all travelling “out”, will all cross each other at some other point on the sphere. If the star was on a flat surface, these beams of light would only ever get further apart; there’s nothing that would ever curve the light back towards each other.

In our Universe, we know that there’s no bending of the light as it comes through space (this is from an analysis of the map of the oldest light in the Universe) beyond what you would expect from gravitational forces. This lack of a large scale bending rules out the spherical and saddle-shaped options, and all that’s left are the ones which can be considered flat. While we can’t observe the entire universe to objectively figure out what the global shape of the entire thing is, we know that on the scales of the observable universe, our Universe is pretty darn flat.

This artist’s impression shows how photons in the Cosmic Microwave Background (CMB, as detected by ESA’s Planck space telescope) are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the Universe. Image credit: ESA and the Planck Collaboration

This artist’s impression shows how photons in the Cosmic Microwave Background (CMB, as detected by ESA’s Planck space telescope) are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the Universe. Image credit: ESA and the Planck Collaboration

How do we know that the Universe isn’t a tightly rolled cylinder? Well, we can’t rule out a gigantic cylinder, but it would have to be so large that we couldn’t ever detect a difference between light going “out” along the length of the cylinder and the light going “around”, because as far as we can observe, the Universe is the same in every direction. If there were a preferred direction, where the Universe appeared considerably younger than in the other direction, then we’d get suspicious of a cylindrical shape. But since there’s no evidence for that, we usually describe our Universe as an unwarped, three dimensional, grid. And with that kind of shape, we don’t expect any of the light from the distant universe to be taking a looping path to show us our own Milky Way.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

Why Is The Expansion of the Universe Always Drawn Like A Cylinder?

Why is the shape of the universe depicted as a cone?


If the universe is expanding with all points moving away from each other, how come it is depicted as cylindrical with the expansion outward mostly in one direction?
This image represents the evolution of the Universe, starting with the Big Bang. The red arrow marks the flow of time. Image credit: NASA

This image represents the evolution of the Universe, starting with the Big Bang. The red arrow marks the flow of time. Image credit: NASA

Originally posted at Forbes!

I have gotten a lot of questions about diagrams of the Universe's expansion. I must say the number of questions on this topic is of great credit to how widely circulated one particular diagram from the WMAP team has been. After my recent article about tracing the Big Bang back to its original location, there was another burst of questions about the setup of this particular diagram:

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image Credit: NASA / WMAP Science Team

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image Credit: NASA / WMAP Science Team

It’s true that while we had a long discussion about how the Big Bang was an even expansion of space itself in every possible direction, the diagrams usually give us a more directional vision of the evolution of the universe.  It’s not just this one diagram, either, though the WMAP image is probably the most familiar – if you’ve seen any of these diagrams, it’s probably that one.

The fundamental issue is that the Universe is an evolving four dimensional entity, and an artist has two dimensions to work with, and compressing by two dimension is really hard to do. Artists are pretty good at compressing three dimensions into two dimensions – we can imply a lot of depth with clever use of perspective.  And in fact the artist who’s constructed the WMAP image is doing just that by giving you a cylinder of space, which we have all successfully parsed as “has some volume”.

WMAP observes the first light of the universe- the afterglow of the Big Bang. This light emerged 375,000 years after the Big Bang. Patterns imprinted on this light encode the events that happened only a tiny fraction of a second after the Big Bang. In turn, the patterns are the seeds of the development of the structures of galaxies we now see billions of years after the Big Bang. Image Credit: NASA / WMAP Science Team

WMAP observes the first light of the universe- the afterglow of the Big Bang. This light emerged 375,000 years after the Big Bang. Patterns imprinted on this light encode the events that happened only a tiny fraction of a second after the Big Bang. In turn, the patterns are the seeds of the development of the structures of galaxies we now see billions of years after the Big Bang. Image Credit: NASA / WMAP Science Team

Here’s the issue: how do you draw and illustrate a changing three dimensional object? You can draw it at different stages, like a biologist’s illustrations of a jellyfish in different stages of life. You could make a video out of it, of course, but if your aim is to make an illustration, you’re stuck with a single image. The other option is to try and take a slice of the whole object, and show how that section evolves over time.  It’s definitely incomplete, but it might give you a better sense of the changes going on, particularly if you can make the assumption that every other section you might have chosen is doing pretty much the same thing.

That’s what’s happened with the cylinder view.  We’ve taken, effectively, a narrow cylinder of current-day space, and shown you how that evolves backwards in time.  In this case, the circular sliver of space that we’re looking at slowly shrinks, and the galaxies that lived in that space in earlier times become smaller and brighter, and less separated, and if we trace that region of space even further backwards, we hit the Cosmic Microwave Background – the oldest light in the Universe.  If we were to keep going, we’d expect this sliver of space to shrink rapidly as we go backwards in time through inflation, and would eventually become infinitely small, as it joins with all other pieces of space we could have selected at the start, in the singularity.  It’s because we’re showing time along the long direction of the cylinder that it looks like there’s directionality here, but in actual fact the expansion is evenly distributed within that cylinder – the expansion of the Universe isn’t “off to the right.”

This diagram, and the others like it are giving you a small slice of the universe to look at, rather than attempting to show the evolution of the entire universe, if such a thing were possible. This is a simplification of how the entire Universe has changed and evolved over time, but you could make a similar slice of any other piece of space that exists today – in tracing it back, you’d see the same sorts of changes.

Have your own question? Feel free to ask! Or submit your questions via the sidebar, Facebook, twitter, or Google+.

Sign up for the mailing list for updates & news straight to your inbox!

Can We Find Out Where The Big Bang Started?

Is there a reason why we can’t extrapolate the expansion of the universe backwards to determine where it all started in the Big Bang? Thanks!
A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image credit: NASA / WMAP Science Team

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Image credit: NASA / WMAP Science Team

Originally posted at Forbes!

Nope – we can, in fact, trace the universe back to where it all started. Unfortunately, it rapidly gets complicated, because the answer is that it began where you are sitting. And also where I am sitting. And also where everyone else on the planet is sitting. And also at the center of our galaxy, and at the center of every other galaxy.

The idea here is that our current universe is expanding, so the universe must have been smaller in the past. So if you take every point in space that exists now, and trace it backwards, all those points get closer and closer together until they reach a mathematical and physical stopping point – a singularity. A singularity is an infinitesimally small point, which can contain quite a large amount of matter or energy – the centers of black holes are also singularities.

The singularity we reach if we trace back the whole universe must have contained all the energy that now exists in our universe, as either mass or light, or dark matter, or dark energy. But it also contained all the space – so all the points of space that we now see as very widely separated were present within that singularity. So the “where” of the Big Bang is, quite literally, everywhere.

We have quite a lot of evidence pointing us to this idea of a very tiny universe at the very beginning of our universe; one of the more important being the detection of the cosmic microwave background (or CMB for short). This background radiation is called “background” because our universe has a fundamental glow in the microwave that you can’t escape – any other observations you’re making at this wavelength will be in addition to the CMB.

Critically, the CMB is very precisely almost the exact same in every direction that we look, and even though this glow is the oldest light in the universe, and the universe was much, much smaller than it is now, you would still not expect it to be the exact same everywhere — unless the universe had been even smaller previously. The theory of the Big Bang produces this naturally, because in between all space being compressed into the singularity, and the production of the light we see as the CMB, there is predicted to be a period of super fast expansion — inflation. Or, if you’re tracing the universe backwards in time, the universe shrinks dramatically down.

The thing to keep in mind with the Big Bang and the expansion of the universe is that it wasn’t an “explosion” like a detonation here on earth, with a definite center, and the universe spooling outwards into a pre-existing space. The closest you can get while thinking of conventional explosions would be if you managed to really effectively explode a tiny object, and then asked “Where was this piece when the explosion happened?” It was at the center, with all the other scattered pieces. For our universe’s expansion, each of those pieces would have to be markers in space itself. Where did the universe’s big bang happen? It happened where the universe was small, and each fragment of our current universe was there to witness it.

Have your own question? Feel free to ask! Or submit your questions via the sidebar, Facebook, twitter, or Google+.

Sign up for the mailing list for updates straight to your inbox!

What if the Universe was a giant tiger surrounded by dinosaur bones?

My son, Mr. 5, keeps asking what’s outside the universe. I asked him if he had an idea. “What if the universe was in a giant tiger and if we had a spaceship with enough petrol we could go outside of its mouth.” I asked him what was outside the tigers mouth. “Bones. Dinosaur bones.” We have since talked about the edge of the universe and he’s unsure of his original tiger idea. Any help?

First off - this is a strictly incredible mental image. I hope you keep writing Mr. 5’s ideas down!

Unfortunately, there’s no indication that our universe is in any way tiger-shaped. The Universe seems to be almost exactly flat in every direction - which means that if we were to get a spaceship with infinite fuel and shoot off in one direction, we would never loop back on ourselves or find the edge of the tiger; we’d just see a lot of universe that looks pretty similar to the portion of the universe we can see from the Earth.

(As a further note- as far as we know, the only dinosaurs that have ever existed were here on Earth. It’d be quite a shock to find more dinosaur bones elsewhere in the universe.)

“What’s outside the Universe” is actually a really hard question to answer, because language often gets tangled up and in the way. The Universe is defined as everything that exists - all the matter, and light, and dark matter - all the stuff that makes planets and people, and all the energy and the space between them. Within the Universe is the only region where “space” and “time” have any meaning.

The second complication is that it’s not mathematically clear whether or not the universe is finite in space. If it is infinitely large, the concept of “outside” the universe means even less - since there’s galaxies and planets for an infinite distance no matter which way you go. There’s no indication that there are fewer galaxies in any given direction, which you might expect if the universe had an edge, or stopped somehow. Often we just assume that the Universe is infinite in all directions because it makes it slightly less mindbendy.

So, (tragically) no space tigers or space dinosaurs, just an awful lot of galaxies.

Something here unclear? Have your own question? Feel free to ask! Or submit your questions via the sidebar, Facebook, twitter, or Google+.