Your Cosmology Questions Answered

Posted by Lydia on June 6th, 2009

We received some very interesting questions on the Astronomy Today Forum last month which I sent to several amateur astronomers whom I consider to be “experts” on the subject.

Question 1: When we look into space we can see back in time some 14 billion years. Can we see this far in all directions? If so wouldn’t that mean we are at the centre?

Question 2: I understand the universe is expanding with our galaxy likened to dots on a balloon. The space constantly increasing between them as the balloon is inflated. So how is it possible for galaxies to collide? A short cut through warped space? Perhaps it should be compared to two or more balloons inside one another, lots of “little” Big bangs. Possibly not all at the exact same location.

Question 3: If our galaxy and Andromeda are going to collide and we were both created at approximately the same point in space shouldn’t a line between them have to point at the approximate centre of the universe? Would a line drawn between other colliding galaxies point to the same location?

Question 4: If everything were created from a single location in a microsecond of time everything should be moving away from that point. After some time that point in space should be devoid of most matter, a big huge empty sphere. Has anything like that been seen?

Question 5: Perhaps with the improved Hubble we will see the edge of space, (moments after the big bang) not the farthermost edge but the edge of that huge empty sphere in the middle. It might appear all around us due to warped space.

Answers after the jump:

Guess who stepped up to the challenge of answering these questions: our colleague, Ms. Brenda Culbertson, and her friend and co-worker, Dr. Brian Thomas Assistant Professor of Physics and Astronomy at Washburn University in Topeka, Kansas. After dedicating their much coveted time to satisfying our curiosity and furthering our understanding of Cosmology, Philosophy and Science, the talented pair have submitted their work to us. Ms. Culbertson reminds me that Dr. Thomas answered most of the questions (quite thoroughly and entertaining I must add …) and she edited his responses to make them more comprehensible to the majority of our readers. (Thank you Brenda!)

Answer to Q1. There are a couple of ways to think about this. First, you can think of yourself as a very tiny bug who lives inside a very, very, very large loaf of raisin bread. Using your amazing bread-penetrating radar, you are able to see that there are raisins in every direction away from you, as a far as you can see in every direction. If your radar (or for us, our telescope) is limited in how far it can see, then it’s impossible for you to say that you live at the centre of the loaf (universe). It could be that you live somewhere near the “edge”, but your instruments just aren’t able to see quite that far.
In reality, most cosmologists think there simply is no edge or middle to see. Another way to think about the problem, which gets at this aspect, is to think of yourself standing on the Earth. If you look all around you, you might think that you’re standing on the centre of a flat plane. Of course, you know that you are standing on the surface of a sphere. Along that surface (not inside, but *along* the surface) there is no center and no edge, even though the surface is finite, not infinite. The problem with this analogy is that it is a 2D surface, while we live in a 3D universe. The same reasoning applies, but it’s much harder (impossible?) to visualize. Regarding this question, consult this video.

Answers to Q2 and Q3. Galaxies deplete and grow during long intervals of time. They are also in constant motion about an axis and, as a whole, through space. Some are in strings or clusters, but they are still moving. We have not seen a spot void of galaxies. While the universe is expanding, on smaller scales, galaxies feel each other’s gravity and are drawn together by this force. Just like our planet is held in orbit around our Sun, galaxies can be held in orbit around each other, or be pulled together strongly enough to collide, merge together, or pass through each other. Milky Way Galaxy and Andromeda are close enough that the mutual gravitational force is pulling us together. But a galaxy a billion light years (or even a few 10s of millions of light years) away feels such a tiny force from our galaxy that its motion with the expansion of space is unaffected, so we see it moving away. This is because gravity weakens quickly with distance (one over distance squared).
Here’s some relevant web material: News article, Animation, Podcast and Video.
See also answer to #1 regarding the centre. Also, galaxies are not oriented in any preferred direction (and yes, people have actually studied this question – Galaxy Zoo data has been used, for instance), so a line drawn between any two galaxies would point in a random direction, even if there was a centre to the universe.

Answer to Q4. This seriously misunderstands the big bang model, but it’s a common misunderstanding. Big Bang cosmology tells us that *all of space* began at once. It was not “inside” anything. The universe is not expanding “into” anything (and this is a problem that the balloon analogy has). Therefore, a better way to think of it is that all the matter, energy *and space* that now exist were once all at a single point, and then space began to grow and objects began to travel farther apart. Again, no centre, no edge – weird!
We do know with extremely high confidence that the universe is expanding. Hubble (the man, not the space telescope!) discovered this in 1929. We also know with somewhat less confidence that the expansion is actually accelerating, not slowing down, and most cosmologists expect it will continue to expand forever. We also know to fairly high confidence that the geometry of space is flat. The accelerating expansion is attributed to some repulsive “dark energy” force of which we currently have no real understanding, other than we can observe the effects on the expansion of space. (There are some alternative ideas here, such as the new oscillatory model of Steinhardt and Turok, and a model that says that the universe’s geometry is actually a dodecahedron, but these are mostly speculative and less well supported by observational evidence.)

Answer to Q5. See #1 again. Most of us think that there is no edge to be seen, no matter the technology, and also no matter the time – no edge at the start, no edge now. Also, recall that looking out means looking back in time. So, we see the universe as it was at earlier times, not as it is now (except nearby).
This video gives a fly-through of our current understanding of what the universe looks like on the largest scales (based on observations of where galaxies are, from the Sloan Digital Sky Survey). The Astronomy Cast podcast is recommended, it covers many different astronomy questions and does a very good job of explaining the hard concepts. Astronomy Notes is also a good general reference ….

Important points to ponder on a dark night as you peer into space from the Blue Planet.

5 people have commented

puzzled said,

Great stuff, very helpful. I can accept that there is no edge to space because it is not expanding into anything. TheBadAstronomer video also says that space does not loop back on itself in any way, so if you travel in a straight line, you won’t eventually come back to the same place. But if there is always more space to explore, doesn’t that make space of infinite extent? I thought it was also settled that space is of finite (though very large) size.

Arcot said,

Thank you Lydia, and a special thanks to Ms.Brenda Culbertson and Dr. Brian Thomas for taking the time to explain and in such a way that I understand! (almost)

I am still unclear about one thing. No center and no
edge, okay. But at some time in the future we may
invent a way to see back to just a few micro seconds
after the big bang. What do we see and is it all around
us? Maybe I should try to imagine infinity or a trillion
dollars first.

Brenda Culbertson said,

I will give your questions some consideration and try to formulate a reply worthy of posting here. I forwarded the additional questions to Dr. Thomas, who probably has very good answers already in mind.

Brian Thomas said,

My impression is that most cosmologists think the universe is infinite spatially.

What we would see all around us at microseconds after the big bang would be a soup of energy and particles. It would indeed be all around us. This is like the cosmic microwave background – we see it all around us and it is a snapshot of the universe at about 380,000 years after the big bang.

John said,

I think perhaps one day in the future, people will look back on us as silly for entertaining the idea of an “edge of the universe,” the same way that we look back on people who thought there was and “end of the world” where your ship would fall off forever.

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