Why Just Us ?

81+

Why do cosmologists limit their thinking and calculating to just our Observable Universe (OU)? I've never seen a single article saying that the sum of everything that exists is in our OU and that there's nothing more "out there" beyond what we can see. In other words, I think that everyone agrees that what we can presently see in our OU is not the total of what resulted from the Big Bang. That being so, there most probably are thousands, perhaps millions, of other OUs just like ours "out there" but they are outside our range of vision at present. These others "out there" may be identical in all ways to our OU (most probable scenario) but they could just as well be a bit different --- maybe even a lot different. Doesn't this mean that these other universes that are beyond our observation could be influencing our OU in some ways (maybe a lot of ways)? Why isn't this rather probable possibility being taken into account? If they were close enough and there were enough of them, could this perhaps account in some way for what we're calling "Dark Energy"?

Frank

PhanthomJay

If you will pardon this response from a non expert, this is similar to what I believe Hawking and the M-Theorists speculate (reference Hawking's "The Universe in a Nutshell"). If there are other universes besides ours, they unfortunately can never be observed, because light and EM waves cannot penetrate the minute extra spatial dimensions of our multi-dimensioned universe (such dimensions may be on the order of the Planck length, much smaller than the wavelengths of even the highest frequency Gamma Rays). Hawking does argue, however, and someone please correct me if i am wrong, that the possibility exists that ultra short wave high frequency gravity waves, if they exists, may be small enough to penetrate in between universes (which may be as close as silly millimeters apart), and such penetration, which would likely be via black holes, may be an explanation for the mysteriious 'dark matter' that fills most of our universe (the penetrating gravity waves from the other universes, which is Energy, shows up in our universe as dark (forever unobservable) matter per the conversion factor E = mc squared). And that further, a collision of universes could be what caused the Big Bang. At least that's my take on it, and even if incorrect, it sure makes for interesting thought discussions.

cristo

PhanthomJay: The OP was talking about the observed universe, not a 'universe' in the speculative sense of string theory/multiverse stuff. The observable universe is the sphere centred on ourselves and bounded by the particle horizon (the maximum distance at which objects can currently be [theoretically] observed).

81+: Of course, there are infinitely many observable universes, some which overlap ours, and others which do not. Cosmologists do not consider what lies outside of our own observable universe for the simple fact that we cannot possibly see it. And, if we cannot see it, then there is no way for such a region to affect our observable universe.

81+

Phanthomjay, I guess I wasn't as clear as I should have been. I'm not talking about other "Total Universes", like in a multiverse. I'm talking about other sections (regions) of our "Total Universe" (the universe that resulted from the Big Bang) outside (next to) the part of the "Total Universe" that we can see at the present time. For instance, if there were an "observer" on a planet that was about 16 billion light years from Earth, his observable universe would overlap ours by about 10 billion years. In other words, he could see galaxies that were only about 3 million years away from us, but that would be the limit that he could see (assuming his technology was exactly where ours is --- a very unlikely assumption because it would only require that they evolved just a few thousand years ahead of us for them to be way, way ahead of us in technoloby)

Frank

81+

Cristo, thanks for the comment as I always enjoy your thoughts -- and humor, though mostly very subtle. However, I'm a bit surprised at your saying:

And, if we cannot see it, then there is no way for such a region to affect our observable universe.

81+

Oops, must have hit the wrong key and posted my reply to Cristo before I was finished. Bummer. I meant to ask how can you say that? There could be huge voids out there right next to our OU, or very, very high density areas either of which could have some effect on our OU. Wouldn't it be at least worthwhile to assume a very dense area and see how this might effect our OU. I'm sure that this has been done. Do you know what the outcome indicated?

Frank

marcus

It IS taken into account. cosmologists are constantly modeling and making statements about the whole universe-----not just the observable piece of it.

The conventional models assume that there is plenty more out beyond the 46 billion LY radius of what we are currently observing.

I am not sure where you get the idea that scientists limit themselves to just modeling and talking about the part of the universe that is currently observable.

But EIGHTYONE, why do you talk about those "other universes"? conventionally it is all one universe and of course other observers would have other regions that they would be able to observe, but it is all one thing. the usual models wouldn't work if there wasnt some assumed uniformity out beyond city limits. They'd have to be more complicated.

You are right, of course it has been done. David Wiltshire is famous for trying to fit the acceleration data by assuming our local region is underdense and surrounded by overdense. Plenty of other people have tried to juggle assumptions so as to avoid the need for dark energy!

The question is how well can you do it. And in the end isn't it more complicated to explain it that way? And what do you do when the next batch of data comes in, showing a new wrinkle that you didn't predict.

So far my reading is that Wiltshire has been trying since about 2005 and it isn't very successful or impressive and people are losing interest. I could be wrong. I think that the dark energy assumption has been winning credibility and support---basically because it continues to fit as more data comes in about the past acceleration history (higher redshift supernovae). But again that is just my impression.

Chronos

Not wrong IMO, marcus. Efforts to model the universe without a dark energy component have done more to strenghthen the case for a dark energy component.

yuiop

A region that is just outside our observable horizon does affect our observable universe gravitationally even if we can not see it. You would not be able to say with any certainty that there other universes out there, some of which overlap ours, if that was not the case. The models that predict the curvature of what we see assume mass outside the visible horizon. The prediction for a spatially infinite universe with infinite mass differ from that of a universe that is finite and only marginally larger that what we actually see. Some experimental surveys have been done that quote lower limit on how much larger the total universe is than the visible universe.

For example, we can probably rule out a hypothetical model that claims our visible spherical universe is enclosed within a slightly larger elongated spheriod because it would make a difference to what we observe ,even though the asymmetry of the mass outside the visible horizon is not directly visible.

[EDIT] It just occured to me that to a civilisation on the edge of our OU would see us as a wrinkle in there CMB because they would be seeing our galactic region as it was just after the big bang.

Chronos

Your thought experiment is flawed. There is no 'edge' to the observable universe. The CMB is the same in all directions to all observers.

PhanthomJay

This statement implies, to me, the existence of gravity waves, and greater than lightspeed travel of such waves, neither of which has been proven. If in fact our observable universe is affected by mass and gravity from beyond the observable horizon, the unobservable universe them becomes observable, by the measure of spacetime curvature in our observable universe. I mean, observable has a greater meaning than 'observable by eye', as the universe still exists when our eyes are closed.. This seems contradictory. Correct?

cristo

For the record, my post #3 doesn't make much sense: I'll retract the second paragraph, and blame it on the time I posted it An important thing to remember is, as marcus says, that we tend to assume some sort of uniformity over the entire universe. Once you do this, then you only really need to consider the observable universe since it is all that is useful for making observations. Of course, once the assumption of uniformity is dropped, then we are playing a completely different game.

marcus

the implication doesn't work, Jay.

kev's statement is obviously correct and implies nothing whatever about any influences traveling faster than light

Like ships at sea each with a 20 mile horizon radius.
I can see the ships that happen to be near me and I will be influenced by their behavior (at least if out of ordinary)

You are, say 15 miles from me. You see and derive information from ships I cannot. In fact you see some ships that could be as far as 35 miles from me. They aren't in my observable, but I may be able to infer that they are there by watching your movements.

Imagine the whole ocean covered with random scattered ships, where each ship has a tendency to gradually flee from empty ocean and slowly gravitate towards higher density regions.

I can tell there must be other ships out beyond my 20 mile horizon because if it were all empty ocean besides what ships I can actually see with my own eyes, then the outermost would all be heading in towards me. But they are just cruising about randomly and not all heading in to me. So I infer the waters are occupied, beyond my horizon, and absent any consistent asymmetric pattern I assume occupied with approximately the same density of ships.

This roughly uniform average density idea is in line with something Cristo said just now:

yuiop

When we look at a distant galaxy we see it as it was, not as it is now, because of light travel time. If an observer was on that distant Galaxy now, looking back towards the Earth they would see the Earth back in time. Further away and they might see where the Solar system is located as it was before it even condensed into a Sun and planets. Further away still they might see the Milky Way just as cloud of gas before it became a proper galaxy. Further away still and they would see the the Milky Way gas cloud as a hot plasma (but it would look cold due to red shift) as it was just after the big bang, but they would not be able to see further back in time than that because the ionized plazma inhibited photons moving freely through space. The Milky Way from their point of view would look like part of the surface of last scattering, otherwise known as the CMB. As we look back towards them from the Earth we too see them as an interesting blob in the anisotropic pattern of the CMB.