Time and the age of the universe

[RXP91]

I'm a physics n00b. I know what I know from Brian Greene (gasp!) and other such horrible popular science authors.

Anyway this has been puzzling me,

- since special relativity means that the closer you approach light speed, the more time slows down (according to another observer, not yourself of course)

- Then does this necessarily mean light from the initial big bang, i.e. photons created during the explosion will never age. So if (theoretically) we could speed past the photons at the furthest ends of the cosmos and look back at them, they'd just be stuck in time, like a photo? So we could essentially see a picture of the big bang forever?

[Fazor]

That's pretty much how I understand it. Of course, once you caught up to the "edge of the universe" you'd haveta match speeds with it for everything to stand still like in a photo.

The other part would be, how would you every catch up to it? If the lights "radiating" out at the speed of, well, light, and nothing can go faster than c, it's kinda a moot point 'caus you'd never be able to get to that point to observe it.

Those are my thoughts, anyway.

[astromark]

Yes it would seem your understanding of this subject is not to far from what might be the truth. You must not call the big bang an explosion. Those people who understand these things seem to not like the inference of an explosion.,but I would like to twist the knife a little on this point. If all of the mater is expanding away in or at the speed c. sort of explosive expansion it is. without the explosion of mater would there be expansion? the word explosion does not require combustion just accelerated expansion, just like a 'big bang'. Can you tell me why is it accelerating?

[Fazor]

Hmm, aren't there many theories for why the universe is expanding? I didn't think that was a known. One question would be, is the expansion speeding up, slowing down, or staying constant? The other probably many people have is with the idea that while space is expanding, the space (or distance) between objects in the same galaxies are not.

Although; hypothetical question. If the universe was collapsing in on itself faster than the speed of light, then wouldn't it be observed as the reverse? (i.e. as if it was expanding instead of collapsing?) Ugh I'd have to go dig up that post here where someone posed the hypothetical situation of a wave moving along a shore-break faster than the speed of light, where it was explained why it would be observed as moving backwards.

[pzkpfw]

Yes it would seem your understanding of this subject is not to far from what might be the truth. You must not call the big bang an explosion. Those people who understand these things seem to not like the inference of an explosion.,but I would like to twist the knife a little on this point. If all of the mater is expanding away in or at the speed c. sort of explosive expansion it is. without the explosion of mater would there be expansion? the word explosion does not require combustion just accelerated expansion, just like a 'big bang'. Can you tell me why is it accelerating?

I think the reason some don't like the word "explosion" is that it implies there is some kind of centre of the Universe where the big bang "occured" and from where all stuff was flung out by the "explosion". It also tends to make it sound like space was already there and the new stuff is just being spread out into it.

However, space itself (the whole Universe) was created in the big bang, and everything is expanding away from everything else - there is no "centre".

I've seen a quote from one astrophyisist that it should have been called the "big expansion" - specifically to get away from the "explosion" implication of "bang".


I hope I'm accurate in retelling what I've read here.

[dvb]

Hmm, aren't there many theories for why the universe is expanding? I didn't think that was a known.

Theory would imply that we have quantifiable evidence in one way or another. Model would probably be a better term. Also remember that why isn't a question of science. One can make fairly accurate predictions through observation, and anything beyond that is merely speculation.

One question would be, is the expansion speeding up, slowing down, or staying constant?

Evidence has been gathered through the observation of type 1a supernovae that the expansion is indeed accelerating. I'm sure a quick google could give you the specifics.

The other probably many people have is with the idea that while space is expanding, the space (or distance) between objects in the same galaxies are not.

The best guess that cosmologists have been able to come up with so far is what we call dark energy. It is assumed that the gravity within galaxies is strong enough to keep things from expanding locally. Dark energy is the force that we've come up with to explain what is causing the expansion between galaxies. Again, I don't know all of the particulars, and google could probably explain things better than I.

[EvilEye]

Well... now this brings up a secondary question.

What is the expansion rate of the universe?

If it is lightspeed... then there is no way you could ever reach the outer edge to check.

[Fazor]

Thanks dvb, I guess I shouldn't have started with an actual question first, as those other questions were more for the OP to think about to go along with astroMark's question. I do appreciate the correction on theory/model, as I never know what to call it when all the ideas are, well, just that.

As for my understanding of dark energy, it's my understanding that this suspected weak force is less than that of gravity, and that's what "holds galaxies together" if you will. Of course, then the question remains, if that is true, then how did galaxies ever get far enough apart so that this dark energy could overcome gravity to begin with? (working on the assumption that the universe started as a singularity).

[antoniseb]

if (theoretically) we could speed past the photons at the furthest ends of the cosmos and look back at them, they'd just be stuck in time, like a photo? So we could essentially see a picture of the big bang forever?

No. There is a lot here that is implied that is naive. The big bang did not emit lots of photons that are gliding out at the edge of the universe holding an image of the initial moment.

[Ken G]

For one thing, in the reference frame of a photon the universe is length contracted into a single point. It's a very ill defined reference frame, because all photons are born and die at one point and one time in their own frame. Nevertheless, this does not alter the sequence of events that occur-- in the limit as you approach the photon frame, the sequence of events that the photon passes through is never compromised. So the "image" of the initial state is quickly lost, even in the limit of asymptotically approaching the photon frame itself.

[Fazor]

No. There is a lot here that is implied that is naive. The big bang did not emit lots of photons that are gliding out at the edge of the universe holding an image of the initial moment.

Ah, that's a good point. Stars didn't appear instantly at the moment of the Big Bang (i still like "Big Expansion" better but not as catchy, i suppose) and there wasn't a "snapshot" of photons or whatever that are now traveling through space. I guess it's just the difference between that model of the universe and what we tend to picture in our heads.

[dvb]

Thanks dvb, I guess I shouldn't have started with an actual question first, as those other questions were more for the OP to think about to go along with astroMark's question.

Ah yes. After rereading your previous post, I wasn't quite sure whether the second question was really a question or not. I just ended up leaving it in there as the information could be useful to someone else.

As for my understanding of dark energy, it's my understanding that this suspected weak force is less than that of gravity, and that's what "holds galaxies together" if you will. Of course, then the question remains, if that is true, then how did galaxies ever get far enough apart so that this dark energy could overcome gravity to begin with? (working on the assumption that the universe started as a singularity).

A good question, and one I wouldn't mind learning more about. My guess is that the initial expansion accelerated the galaxies beyond escape velocity of all but their neighbouring galaxies that shared the same inertial frame of reference. But I really have no idea, and corrections are welcome.

[Fazor]

A good question, and one I wouldn't mind learning more about. My guess is that the initial expansion accelerated the galaxies beyond escape velocity of all but their neighbouring galaxies that shared the same inertial frame of reference. But I really have no idea, and corrections are welcome.

That would be my guess too. It's actually not really a mystery (not compared to what caused this initial expansion, and those kinda questions). Just like if you have a pile of magnetized metal shavings, and you drop a heavy ball on them (okay drop anything heavy on them) they'll skatter away from the force, but as they slow down clumps will form from the shavings close enough to still be attracted.

Basically I guess the only real conclusion would be if darkenergy is weaker than gravity, then it's apparent that it was not dark energy responsable for the "big bang", it had to be some other force. Or at least, that would be my conclusion.

[Cougar]

What is the expansion rate of the universe?

From the WMAP site:

By characterizing the detailed structure of the cosmic microwave background fluctuations, WMAP has accurately determined the basic cosmological parameters, including the Hubble constant, to better than 5%. This measurement is completely independent of traditional measurements using Cepheid variables and other techniques. The current results show the Hubble Constant to be 73.5 +/-3.2 (km/sec)/Mpc. If the WMAP data is combined with other cosmological data, the best estimate is 70.8 +/- 1.6 (km/sec)/Mpc. These results assume that the universe is spatially flat, which is consistent with all available data. However, if we relax this assumption, the uncertainty in the Hubble constant increases to +/-4 (km/sec)/Mpc, or slightly over 5%.

So the expansion rate is around 70 km/sec for every megaparsec (which is about 3.26 million lightyears).

So proportionately, if my arithmetic is accurate, the Hubble expansion rate over a distance of ~3 lightyears (which is still pretty far) is 0.00007 km/sec. (That's about 3 inches.)

[speedfreek]

The problem with thinking of the expansion of space as coming from an explosion is that an explosion has a source point, a centre, whereas the expansion of space is an expansion where the metric that describes distance is changing over time. This means there is no centre of expansion, as everything is expanding away from everything else.

The metric expansion of space is unlike any other kind of expansion we know. Everywhere, at any given time, is expanding at the same factor. What this means is that over a given time period, if 1 meter expands to take up the distance that 2 meters used to take up, in the same time period 1 million km expands to the distance 2 million km used to be, and thus the universe doubles in size. This is how we can have galaxies receding from us faster than light.

[danscope]

" And far past what you could imagine......there is still more, and more beyond that........"

[jamini]

I'm a physics n00b. I know what I know from Brian Greene (gasp!) and other such horrible popular science authors.

What's your problem with Brian Greene?

[Fazor]

I haven't gotten to read any of his work yet, but the reviews I've heard have all been very positive for Brian's stuff. I do have one of his books on my amazon 'wish list', but I figure I should first re-familiarize myself with the basics before I go reading something any more indepth.

(PS, Don't you like how I refer to him as Brian as if I know him as more than a name on the cover of a book? for the record, I do not).

[dvb]

What's your problem with Brian Greene?

Well, he is a string theorist, and not all scientists agree that string theory is a science. I couldn't see any reason beyond that though, and I can't say I know much more than that about him myself.

I don't hold any judgements against him.

[Sam5]

- Then does this necessarily mean light from the initial big bang, i.e. photons created during the explosion will never age. So if (theoretically) we could speed past the photons at the furthest ends of the cosmos and look back at them, they'd just be stuck in time, like a photo? So we could essentially see a picture of the big bang forever?

This is often misunderstood. When you see a photo of stars, they aren’t moving, and you aren’t seeing their photons. You are seeing a continuous stream of photons from the light in your room or from sunlight outdoors as it is being reflected off the photo and into your eyes. The stars aren't moving in the photo because the photo was exposed over a short period of time, let's say one minute, and those original photons were absorbed by the emulsion of the photo negative. So while watching the photo, you are seeing a representation -- a photo -- of that one minute over and over again, but you aren't seeing any of the original stars' photons.

If you could go out in the universe and look back you wouldn’t see the same photons over and over again, and, therefore a “still picture” of the universe in the early times. A photon is absorbed as soon as you see it. It doesn’t continue to wiggle in your eyes so that you see a “still photo” type image.

You can think of it like this: If you could rush out 50 light years from earth, then turn around and turn a sensitive TV antenna toward the earth, you could see the old “I Love Lucy” show being broadcast “live”, but that’s only if you go out and then stop. If you continue to go out at the speed of light you won’t see anything, since the radio/TV waves from the show would be infinitely redshifted. You can’t continue to see the same waves over and over again and see the show as a still frame. Light doesn’t work that way.

[RXP91]

What's your problem with Brian Greene?

His popular science books have been accused of distorting physics from the general physics community.

[EvilEye]

This brings up an interesting conundrum...

Brian vs. NASA, vs Kaku, vs Hawking.

No 4 of them agree with at least 2 of the others.


So what's right?

[jamini]

His popular science books have been accused of distorting physics from the general physics community.

Can you please provide some specific examples of Mr. Greene “distorting physics from the general physics community”.

I have read all of Mr. Greene’s books and his interpretations of Newtonian laws, SR/GR, QED and QCD all seem to agree 100% with mainstream science.

[Cougar]

I know what I know from Brian Greene (gasp!) and other such horrible popular science authors.

Dr. Brian Greene is probably one of the better popular science authors, and I've read a few , including Greene's The Fabric of the Cosmos; Space, Time, and the Texture of Reality. He's not just a science writer. He's a professor of mathematics and physics at Columbia. There are better science authors who write for a general audience (my opinion), but I would recommend Greene before Kaku and even Hawking.

When writing for a general audience, it is simply necessary to simplify things. Publishers tell such authors that they must limit or exclude equations from their descriptions. This is not exactly "distortion," but you can't exactly tell the WHOLE story now, can you? Another prof. (at Harvard) said...

"More and more scientists are now engaging in the popularization of their own fields. I wholeheartedly support this trend for the simple reason that scientists know their turf better than journalists. At the same time I am disturbed to see a growing gap between the standards upheld by scientists when they face other scientists and the standards they uphold when they face the public... [saying] things they would never try to get away with among colleagues." -- Tony Rothman

I don't think he's talking about the better writers like Greene when he says he's "disturbed." I think this is directed at exclamations to the media such as, "It's like seeing the face of God," and similar meaningless quotes.

[Cougar]

...not all scientists agree that string theory is a science.

The authors of the thousands of scientific papers on aspects of string theory would probably be surprised to hear this. But I understand the "controversy." For a full treatment and thorough support for the validity of pursuing this area, see The Cosmic Landscape by Leonard Susskind.

"Physicists are not like ordinary people, and string theorists are not like ordinary physicists."

[Fazor]

...not all scientists agree that string theory is a science.

You'd probably be hard pressed to find any theory that is supported by all scientists. But if you mean there's a good percent that don't (I haven't studied string theory/SR/GR so I don't know what the percent is) you're going to have to come up for a better argument as to why this makes him a bad scientist. ANY scientist on the leading edge of a particular field is not going to be in agreement with a portion of the scientific community. If you stayed within those "conventional bounds" not too many new discoveries would be made. Do you think Relativity was mainstream ever since it was proposed and then refined?

[SharkByte]

A good question, and one I wouldn't mind learning more about. My guess is that the initial expansion accelerated the galaxies beyond escape velocity of all but their neighbouring galaxies that shared the same inertial frame of reference. But I really have no idea, and corrections are welcome.

My thoughts on the matter are that initially, there were no galaxies. I imagine the big bang as something similar to a supernova only on a much, much, much bigger scale. For all intents and purposes, it started with a singularity containing all the matter in the universe which went supernova flinging matter in every direction. Over the next few billion years that matter started to clump together and eventually formed the galaxies, stars, planets and all the other stuff we see in the universe now. Galaxies never had to reach escape velocity of all thier neighboring galaxies that shared thier same inertial frame of referance because they were already moving at that speed when they formed and became galaxies.

I have always wondered, if you could get outside the universe far enough away to see the whole thing, what would it look like? I have always imagined that it would look something like the Southern Ring Nebula (NGC 3132) or the Hourglass Nebula (MyCn18).

[EvilEye]

My thoughts on the matter are that initially, there were no galaxies. I imagine the big bang as something similar to a supernova only on a much, much, much bigger scale. For all intents and purposes, it started with a singularity containing all the matter in the universe which went supernova flinging matter in every direction. Over the next few billion years that matter started to clump together and eventually formed the galaxies, stars, planets and all the other stuff we see in the universe now. Galaxies never had to reach escape velocity of all thier neighboring galaxies that shared thier same inertial frame of referance because they were already moving at that speed when they formed and became galaxies.

I have always wondered, if you could get outside the universe far enough away to see the whole thing, what would it look like? I have always imagined that it would look something like the Southern Ring Nebula (NGC 3132) or the Hourglass Nebula (MyCn18).

The current belief is that the universe began from the big bang... a point of unimaginable smallness, and exploded into a cloud of energy that expanded to the size of a grapefruit in a million years.

Then It began to combine into helium and hydrogen and a little other stuff...

Those new atoms had gravity and they started to attract each other.

They became stars.

The stars turned on and forced atoms to make other elements.

The stars died and spewed these new elements into space.

Those elements became planets, and the things we find on them.


A little simplistic..yes...but effective.

[01101001]

The current belief is that the universe began from the big bang... a point of unimaginable smallness, and exploded into a cloud of energy that expanded to the size of a grapefruit in a million years.

Or a trillionth of a second. Or a thousandth of a second. Or 10^-31 second.


A million years seems a bit lethargic.

[EvilEye]

Or a trillionth of a second. Or a thousandth of a second. Or 10^-31 second.


A million years seems a bit lethargic.

LOL splitting hairs over time is kind of silly when time was just being created isn't it?