How do we know the universe is expanding?

Emptiness is probably infinite, although we have no way of proving that other than by thought experiment. Spacetime is defined as distance between things. Nothingness is outside the concept of spacetime, but you are quite correct, there is no circle as shown in the two diagrams. That was probably just an unfortunate convention the artist used to group the galaxies together.

The Universe could be infinite, but we can’t observe enough of it to definitively say. We do know it has a radius of at least 70 billion light years. We also think that while all the other galaxies seem to be rushing away from us, with the most distant one going fastest, leaving the impression we are at the center of the Universe, that it would actually appear that way no matter where you go in it. So either is is infinite or spacetime is non euclidean and has more than 3 spatial dimensions.

The expansion of the universe is established by measuring what’s called the redshift of galaxies near to us and others further away. We find that the more distant a galaxy, the greater its redshift, therefore the greater its speed of expansion away from us.

What is expanding is spacetime. It isn’t like everything is expanding equally. The Earth isn’t getting further away from the Sun because of expansion of the Universe. The stars of our Milky Way galaxy aren’t getting further from its center. But the spacetime between our galaxy and all other galaxies is growing. And the growth gives the appearance of the most distant objects from us rushing away from us at greater than the speed of light.

Now, here is a thought to toast your noodle. If the universe is infinite, what is left outside of it for it to be expanding into?

So really what they mean when they say the universe is expanding, is just the distance between galaxies is growing?

I read that old question of yours just prior to asking this one. That has also bothered me in my thinking of this problem. :P

@uberbatman That’s correct. The distance between cohesive objects such as galaxies, quasars, pulsars, and nebulae is growing, but the distances within cohesive objects is not growing. That much, your two illustrations got exactly right. Hey, and I am delighted that previous question sparked a further interest in cosmology. When we look up at the nighttime sky far from city lights, even with the naked eye, we are awestruck. It is like standing in the presence of the divine. But as we probe deeper into cosmology, it’s just like Alice said. “Curiouser and curiouser!

ahhhh i see, that makes perfect sense. So I was just misinterpreted what was meant by the universe is expanding. Thanks for clearin that up.

Yea space is truly fascinating. I spend much of my free time endlessly wondering about space and all the possibilities.

Is it possible that more dark matter is occurring?

To say the universe grows in size is to say it has a definite size….
it is to say that its subjected to time…
it is to also say it suspended into something greater than itself,,,what ever that is..

None of this is true

The universe increases only in its glory…
which means the perfect is increasing in perfection,,,,.

@6rant6 More visible matter may be occurring ex nhilo, so I don’t see why dark matter couldn’t follow a similar path. Energy/dark energy as well. It is theorized that quantum fluctuations in apparent nothingness—the space between random hydrogen atoms in deep space—can cause particle/anti-particle pairs to poof into existence out of nothingness. Here’s lab work on duplicating the conditions.

Usually, in free space, the particle pair’s own mass attracts them back into one another after a short time, and they then self annihilate—releasing ex nihilo energy. But when the process occurs at just the right distance from the “event horizon“http://en.wikipedia.org/wiki/Event_horizon of a black hole, the particle or anti-particle heading away from the event horizon may escape while the one heading toward it gets pulled into the black hole.

@kess For someone who is constantly trying to sound so profound and spiritual, the postulate that a finite universe would have to be suspended in “something” is an incredibly limited piece of thinking.

The distance from us to everything is increasing. If everything occupied the same space then some things would have to be closer to us.

maybe people just think that because the moon is travellin further away from the earth every year…

Because it gets quite dark after the sun sets. See

http://en.wikipedia.org/wiki/Olbers'_paradox

“The argument is also referred to as the dark night sky paradox.”

If you imagine the universe as a static size, then everything should be moving randomly unless bound by one of the four forces. If the motion of the galaxies was random, there would be a near equal number moving towards us as away from us. However we observe that nearly all galaxies are moving swiftly away from us. The only two solutions to this problem are that some force is driving or has driven everything apart, and we are in the centre, or that the fabric of space-time itself is expanding, so every observer would see the same effect. Since the probability of us being in the dead centre of over 100 billion galaxies is infintesimal, space-time must be expanding in the voids between our galaxy and others outside our local cluster.

@ETpro You still do not have a clue as to what nothingness actually is…....

So therefore, the confusion is yours,,,, but I do understand why you make it appear as mine….cause there is no other way it can ever be…

Anyway Thanks for the many compliments

From the short time that man has had the ability to even see some of the Universe it is only a conclusion with no proof of anykind other than an observance of a trillionth of a second in time. Mind you not a right/wrong one but an observance non the less.

your Mom’s expanding…

Here are my thoughts about “the universe is explanding but everything is traveling towards one point”

Our universe as we know it is actually inside of a blackhole. It appears we are expanding but everything is actually getting smaller as we travel towards the singularity. The “big bang” was actually our universe (or the elements prior to our universe) entering the blackhole. We can’t see beyond this blackhole we are in because nothing can escape it.

It always amazing me that we have to put limitations on the Universe. The big bang is just a theory itself. We all belong to life and the Universe and we live in Universal law which the worlds are governed by. There is nothing to say that we are not just a cell inside a body of a individual living on a world not unlike our own.

@Summum I don’t think we’re trying to put limitations on the universe. We just want to understand it. It’s a very interesting time to have an interest in cosmology.

I think it is wonderful that mankind searchs the Universe for answers but what I mean by limitations is trying to say that this is how it was created and so many years ago. The Universe is like this or that including saying that a planet would have to be like ours to sustain life. None of these are facts they are guesses and fun ones at that. Man needs to constantly look for the answers because they are all there we just have to uncover them. But the laws of the Universe are in place and cannot be changed or broken.

@Summum This “Sustaining life” notion is all yours, not previously part of this thread. I think maybe you have another issue related to that discussion. Why not post a question?

@kess I am sorry if you took offense at my suggestion that you may have missed something about cosmology. After writing that comment, I though I should reword it as it sounded critical and abrasive. Unfortunately, it was too late. Suffice to say I freely admit I do not know what nothing is. I asked about it here because I knew that I do not know. The odd thing is I have learned much more that turned out to be true from those who freely admit not knowing things than from those who claim their understanding is perfect.

@Summun FYI, big bang is a scientific theory… Not to be confused with the literal meaning of “theory.” The big bang is a scientifically proven fact. It’s just as real as e=mc2.

@Silence04 There really are NO scientific proved facts. Science says based on what can be tested over and over that the results if constant is how it seems to be. Scientific fact for your information is constantly changing as we learn more and more. So no the Big Bang is a theory.

There is also a theory of gravity, but I would still strongly advise you not to jump off a tall building, unless you insist on making this silly argument.

…There’s theory of how gravity works. We feel gravity daily, there’s no disputing its existence, just why it exists.

As far as I’m aware, although the big bang is the most believable theory (to many,) there is no actual proof of whether it happened. It’s not just lacking the why, as gravity is. There’s a key difference between the two, in that instead of seeing the what (as we do when we jump or drop things) we’re speculating the what based on its after-effects. Some even theorize there was no beginning of the universe. Much harder to comprehend, and I think less supported by the majority, but not any less feasible evidence-wise.

The idea behind science is to never declare absolutes—this allows the constant in-flow of new knowledge.
There are some theories that have been around for so long, and during that time no counter-evidence has been presented, while supporting evidence continues to be provided, that they’re more or less accepted as true. In this way, there is some ‘fact’. However, all of these carry the fine-print caveat that they could still be proven wrong; which is why they maintain the title of theory: they’re past hypothesis, but not absolute.

The simplest intuition about gravity still requires an assumption that what has happened in the past will happen again. There is no guarantee. All of science hinges on this same assumption. The Big Bang theory is a similar, though more sophisticated, chain of reasoning based ultimately on observation. In this case the key observation is background radiation. It provides very strong evidence that our neighborhood of whatever it is that is out there, came into existence several billion years ago.

@Summum:

I strongly suggest you read The Relativity of Wrong by Isaac Asimov. Scientific theories are never correct. If they are totally correct, they are no longer called theories. However, simply being ‘not correct’ does not equate with being wrong. Scientific theories are not correct in that they still have wrinkles that need to be ironed out. General Relativity is correct in all its predictions to an accuracy of one part in one trillion, but because of its conflicts with other equally accurate theories, we know it is not correct.

However, despite not being correct, the term ‘theory’ carries no negative connotations. As used in the lay sense, a ‘theory’ should be called a ‘hypothesis’, i.e. a probable idea not yet supported by a large body of evidence. For a hypothesis to make it to the esteemed status of ‘theory’, it must be supported by evidence that makes it certain that the basic ideas are correct, and near certain that the details are also hugely accurate. We may discover that a species takes a different place on the evolutionary tree than previously thought, but that should not prompt us to question the validity of Natural Selection. The theories you refer to are far from guesses, they are near certain models of how the universe works.

So no, the Big Bang theory is not a fact, but it is so consistent (when combined with Inflation theory, and all the other tweaks made along the way) with observation that it is highly unlikely to be wrong. Even if it were shown to be wrong, the theory that replaces it would bear all the same hallmarks, and would merely modify whichever details were brought into question.

@FireMadeFlesh I have no problem with all you have stated.

@Summum In that case I have misunderstood you, and I apologise for that. Above, where you referred to guesses, it seemed to me to indicate some level of scepticism in the validity of the scientific process.

No I believe in the scientific process though we are very limited in what we can use it on. We cannot apply it to things that are not physical such as the spirit or our subconscience.

@Summum Thanks for clearing that up, but I must disagree with you there. It is perfectly fine to apply the scientific process to everything, but its success may be limited in some areas. We could use the scientific process to investigate our subconscious, but it would be from a materialist point of view, and would not give us the whole picture. I don’t think we should write off any particular area of interest as inaccessible to science – we should just be prepared to use other methods to compliment it.