What's our universal address?

If we were trying to reach out to somewhere outside of our universe, what would our overall universal address be? The closest I’ve come is this: (personal address), state. USA North America, Earth, Solar System, Global Cluster, Milky Way, Local Group, Virgo Cluster.

This is a surprisingly tricky thing to work out, but the difficulty lies in being able to locate ourselves in three dimensions on large scales more than anything else.

Your addressing scheme (as far as you’ve written it) is pretty accurate up to the Virgo supercluster, which itself is really a conglomeration of other clusters, both small and large, shown above. (This is distinct from the Virgo Cluster, which is about 65 million light years away from our local group.) But that address would really only help people if they know where to look for the Virgo supercluster. The next step up from the supercluster is a loose affiliation of other superclusters, so if you could get everyone in that supercluster affiliation to look at the right supercluster, they might be able to zoom in from there and find us using your address.

We have dealt with this problem on a much smaller scale before. Both the Pioneer and Voyager spacecrafts tried to relay our position in the unlikely event that they were ever encountered and picked up by another intelligent race of beings. The Pioneer plaque showed the location of our planet within our solar system, but it (and the Voyager Golden record) also gave a map that would be useful to someone else within our galaxy. The set of lines radiating out from a single point on the left hand side of the Pioneer plaque and the Voyager record is this map. This gives the positions and frequencies of a set of pulsars - incredibly rapidly spinning objects that beam light out into the cosmos like a lighthouse. Each pulsar has its own unique frequency - the number of times per second it flashes in our direction. Pointing out the distances and frequencies of 14 pulsars, as we’ve done here, should allow someone else, observing the same set of pulsars, to triangulate our position.

However, this can only be scaled up so far. If you’re trying to tell someone outside our universe where we are - or really, anyone sufficiently far away in our own universe - you swiftly become tangled in a different problem, which is that the more distant from our planet you would like to map out for our faraway friends, the more you travel back in time. At some “distance” from our planet, our exploration becomes one much more of time than of space. Even just the Virgo supercluster (a tiny corner of the universe) is some 110 million light years across. Undoubtably, between light leaving that side of the cluster and our receiving it, things have changed over there in the intervening 100 million years. If our visitor happened to be at a 90 degree angle to the line that connects us to the other side of the cluster we observe, they would receive light from both of us at the same time. This means that particular observer wouldn’t have the same time delay that we observe between our current time and the time that light left the other side of the cluster. They would have a different, but equal, delay on their observation of both of us. They might see a structure that looks a little different from the way we understand it, as a result.

This becomes an increasingly disruptive problem the further back in time we go. At some point, we are looking at structures and objects that no longer exist. Many things can happen in a couple billion years, and they usually do, so using those objects on our map isn’t very useful for an inter-universe traveller.

So what can we use as a universal reference frame? Unfortunately, one of the fundamentals tenets of cosmology tells us that there are no “preferred” or “special” perspectives on the universe. This means that there’s no overall zero point everyone can agree on, which makes it difficult to give directions. I’ve talked about this total lack of a universe-wise reference frame once before, and there’s no easy solution at hand.

The best thing we may be able to do right now is to create a really good map of our little local set of superclusters, and tell our visitors to stop by if they happen to see something that matches it in their travels.

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How many galaxies are there?

(Nasa.gov may be dark right now, along with all of its twitter accounts, but Astroquizzical is still going! Remember to submit your questions via the ask page, the sidebar, Facebook or twitter. Apologies in advance for broken links throughout the blog during the US government shutdown.)

To borrow a phrase from Carl Sagan - there are billions upon billions of galaxies out there.

The image above is the Hubble Ultra Deep Field, and I encourage you to look at the high resolution version and pan around. Every single bright pixel in that image is a galaxy (with the exceptions of the spiky ones - those are stars in our galaxy that got in the way). This image alone contains 15,500 galaxies, and it’s a tiny, tiny fraction of the night sky. It’s about a 1200 times smaller than the area blocked out by the tip of your little finger held at arm’s length.

The patch of sky imaged by the Ultra Deep Field was chosen because it was particularly dark. If you want to look back towards the dawn of the universe, at the most distant galaxies possible, you need to avoid looking in a region of the sky with very bright nearby objects in the sky. For us, our biggest problem is avoiding our own galaxy. Certain parts of the sky are completely unusable for studies of distant galaxies; the gas and dust in our own galaxy is simply too bright, and blocks out the light from more distant things. We also have to avoid other nearby galaxies for the same reason - we want to be looking at the blackest, emptiest part of the night sky we can find.

When looking at very dark parts of the sky, an extraordinary amount of time had to be dedicated to acquiring this image. The total amount of time Hubble spent staring at this patch of sky was one million seconds - if this had been observed in a single session, it would have lasted for a little over 11 and a half days.

Since the universe is roughly the same in every direction we can look, we expect the universe to roughly look like the Hubble Ultra-Deep Field in every direction, if we had the ability and the time to look in every direction in the sky. To start to get a handle on how many galaxies this is; a thousand of these images fit under your little fingertip. In any direction you care to face in the night sky, raise your smallest fingertip - you’ve just blocked out the light from over 18 million galaxies.

Have your own question? Something here unclear? Feel free to ask!

Does space go on forever?

Read More

If the universe is expanding, how can galaxies collide?

The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. Image credit: ESA/Hubble

The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. Image credit: ESA/Hubble

This apparent contradiction comes from the way that scientists commonly explain the expansion of the universe; we say, “Imagine a balloon with a series of dots on the outside of it. Now inflate the balloon. All the dots move away from each other as the balloon, which is space, grows in size.”

Or, “Imagine you have a loaf of bread with raisins in the surface. As the dough rises, the raisins will spread further apart from each other.”

This is an accurate metaphor- the fabric of space is expanding as the universe ages. However, when we make these metaphors, we draw out our objects in space - the dots on the surface of the balloon, or our raisins in bread dough - in regular patterns. We put everything on a grid, so that the effects of the universe’s expansion are easier to spot. This is convenient because it means we’re only looking at one effect (the expansion), but it’s a big oversimplification of the universe.

Objects in the real universe aren’t laid out on a grid. The universe doesn’t do grids. Real galaxies are scattered randomly across the fabric of space, which means that sometimes you’re going to wind up with one or two or 50 galaxies pretty close to each other. Sometimes you’ll wind up with a galaxy with nothing around it at all.

When you have two enormously massive objects relatively close in the universe, another force takes over. Gravity. Gravity is an extremely powerful attractive force, and if two objects are near enough to each other to feel the gravitational pull of the other galaxy, it doesn’t matter that the universe is expanding; it’s not expanding fast enough to counteract the attractive force of gravity, and these two objects are going to fall towards each other. When they do, there’s a good chance they will eventually become a single, larger galaxy, and the process gives us magnificent images.

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