Is Time Travel (going backwards) Possible?

[Nellisir]

The simplest possibility is that it is a constant energy density. What this means is that, as space expands, each cubic centimeter always has the same energy in it, so the "total energy" always increases!

So that's where the time travelers are going....

 

[tomBitonti]

Within the context of general relativity, expanding space does not necessarily require work or reduce the total amount of energy. For example, normal matter scattered through space causes space to expand (in a way that slows down as the expansion of space spreads the matter out). In this case, if you were to define a region bounded by some set of galaxies, you'd find that the total energy in that region stays the same over time but gets less dense because that region has more volume.

I've always wondered about that ... does expansion have no effect on a large but gravitationally bound system such as a galaxy? If it doesn't, how are odd effects (anomalous curvature) prevented when transitioning from a uniform expansion in deep intergalactic space to a uniform lack of expansion inside of a galaxy? But if expansion affects the region inside of a galaxy, that would be moving stars every so slightly but gradually further apart, in doing adding energry to the stars.

TomB

 

[Umbran]

For example, normal matter scattered through space causes space to expand (in a way that slows down as the expansion of space spreads the matter out).

Stop right there.

There is no evidence that normal matter itself causes expansion. My understanding is that stuff with positive energy density, like normal matter *cannot* be responsible for such expansion, and having normal matter actually cause expansion is, so far, apparently unnecessary to explain the phenomenon. Much of the rest of what you say is close enough to current models, but this one bit is contrary to the current observations.

I guess to be fair is that there's one other possibility, too. That possibility is that what we think we see in the measurements isn't correct and that the universe is actually slowing down. So you have to give an argument about how that might happen.

Might as well say you have to have an explanation for the possibility that, while I see an apple on my desk, can pick it up, and bite it, there actually *isn't* an apple on my desk.

You don't have to have explanations for things that aren't observed. We observe an acceleration of the expansion. We have double checked those results, corroborated them with several different research groups using different methods. You don't have to explain how all of them might be wrong, until you see evidence they are, in fact, wrong.

So it's not like there aren't explanations beyond description. The problem is more like having too many, and we will need more data to distinguish among them

Yes, and that's exactly my point. You can make up an oodle of cute stories about how the Universe works. Data comes along and shows most of those stories are not consistent with reality. Eventually, you have enough data to prove that all but a very small number of those stories are not consistent with reality. This is where explanation and description become one and the same.

For "dark energy" we are nowhere near discarding most of the stories. We only have a small amount of relevant data. There are a ton of still possible explanations, but only a very small amount of description of what's happening. The two are by no means one and the same.

For example, if you like the cosmological constant, why does it have such a small but nonzero value? That's a bit trickier, worth a whole lot more than one post, and frankly controversial in a number of ways.

No, it isn't. At least, it is no trickier than why anything has its one particular value. It is no trickier to explain this value than to explain the charge of the electron, for example. That there is something weird about it being small is a figment of human perspective and ideas of scale, to which the Universe is by no means beholden.

I've always wondered about that ... does expansion have no effect on a large but gravitationally bound system such as a galaxy?

Not no effect. Just so small that it doesn't impact things.

Within a galaxy, the density of matter is very high, so gravitation dominates over those distances. The repulsion is there, but within a galaxy, the attraction of gravity is much stronger. So you have a strong pull, and a weak push, and the pull wins.

Out in the great vastness between galaxies, however, the normal matter is very, very thin, and the energy associated with the vacuum dominates, so the repulsion dominates.

There's nothing at all odd about this - take a bar magnet, and pick up a pin with it. Locally, electromagnetic forces are overcoming the force of gravity on that pin. Gravity is still there, but it isn't enough to pull the pin away from the magnet.

If it doesn't, how are odd effects (anomalous curvature) prevented when transitioning from a uniform expansion in deep intergalactic space to a uniform lack of expansion inside of a galaxy?

We aren't talking about sudden, sharp transitions. There isn't some sharp wall at the edge of the galaxy, or something. There's just a slow, smooth transition from the area around galaxies out into the deeps.

 

[tomBitonti]

We aren't talking about sudden, sharp transitions. There isn't some sharp wall at the edge of the galaxy, or something. There's just a slow, smooth transition from the area around galaxies out into the deeps.

Yup, I would expect a smooth transition. (Although, the curvature changes seem small. Could the transitions be discrete after all?)

If the effect is as you imply -- present in (mostly) empty intergalactic space but absent inside of galaxies -- wouldn't that make for a "stretched" region of transition, with detectable affects on light passing through the stretched region?

Thx!

TomB

 

[Umbran]

Yup, I would expect a smooth transition. (Although, the curvature changes seem small. Could the transitions be discrete after all?)

Insofar as these days we expect everything to have a representation in quantum mechanics, yes. Otherwise, no.

The force of gravity does not have a discrete cut-off with distance. Neither does the "force" that results from the vacuum energy - both are continuous functions. The sum of two continuous functions will be continuous, with no discrete discontinuities.

If the effect is as you imply -- present in (mostly) empty intergalactic space but absent inside of galaxies -- wouldn't that make for a "stretched" region of transition, with detectable affects on light passing through the stretched region?

No. As Freyar noted, the vacuum energy (the simplest form of explanation, so we'll use that for explication purposes) is uniform throughout the entire universe. It is a result of space itself. Space is everywhere, the energy is everywhere, even within galaxies, and so it has a uniform effect *everywhere*. It is an even background, on which locally sometimes gravity weighs in.

What we do see is its effect on the rate of change of expansion of the space: not only is the universe expanding, but it will be expanding tomorrow just a tad faster than it is expanding today.

 

[tomBitonti]

No. As Freyar noted, the vacuum energy (the simplest form of explanation, so we'll use that for explication purposes) is uniform throughout the entire universe. It is a result of space itself. Space is everywhere, the energy is everywhere, even within galaxies, and so it has a uniform effect *everywhere*. It is an even background, on which locally sometimes gravity weighs in.

What we do see is its effect on the rate of change of expansion of the space: not only is the universe expanding, but it will be expanding tomorrow just a tad faster than it is expanding today.

Then wouldn't you expect the space inside a body such as a galaxy to expand? For any small period of time the effect would be unmeasurable, but over time the effect would show as a continuous expansion (perhaps which is immediately consumed as a contraction and a small addition of energy).

I think that I've heard before that this doesn't happen. if it doesn't, why not? What is wrong with my naive approach that is creating this problem?

Thx!

TomB

 

[freyar]

Then wouldn't you expect the space inside a body such as a galaxy to expand? For any small period of time the effect would be unmeasurable, but over time the effect would show as a continuous expansion (perhaps which is immediately consumed as a contraction and a small addition of energy).

I think that I've heard before that this doesn't happen. if it doesn't, why not? What is wrong with my naive approach that is creating this problem?

Thx!

TomB

The difficulty is that the effects of the overall expansion of the universe (whether the accelerating kind or a more mundane decelerating kind) do not just "add" with the effects of the matter in the galaxy on gravity. (We say the equations of general relativity are "nonlinear" because the results are not additive.) So, while I don't have the exact solutions to the equations at hand in this case, what you do expect to happen is that the gravitational attraction of the matter in the galaxy to "win" over the expansion of the universe, so the space inside the galaxy shouldn't stretch. In other words, the galaxy is "gravitationally bound," so the distance across the galaxy shouldn't be increased by the overall expansion of the universe. That will only affect distances between objects that aren't bound together.

Another point to make is that, even if the expansion of the universe were happening in our galaxy, it would be a very small effect. The expansion is only noticeable on very large distance scales (for example, the expansion rate between our Milky Way and Andromeda, the closest large galaxy, is quite slow).

What's interesting is when you get borderline cases, like galaxies that are gravitationally attracted to each other but also being swept apart by the expansion of the universe. Then there is a legitimate competition between the two effects.

I hope this doesn't muddy the picture, but cosmology is a big and sometimes complicated subject.

 

[Umbran]

Then wouldn't you expect the space inside a body such as a galaxy to expand?

No. Remember above, with the bar magnet? Once the magnet has picked up the nail or pin, do you expect gravity to slowly, very slowly, pull the pin away anyway? No. It's there, it is stuck, and it isn't gonna move.

The forces of gravity are strong near matter. Very close to matter, space isn't expanding at all (very close to really big chunks of matter, space collapses - black holes!). As you move away from matter, though, the force of gravity decreases. Eventually, the force of gravity is small enough that it no longer counters the effective forces of the dark energy, and way out there, expansion accelerates.

In fact, the universe is mostly empty space - there's a whole lot of "force of gravity is small" out there. So, on the whole, the universe is expanding, and that expansion is accelerating. It is only near these tiny islands of galaxies where this is not true.

 

[KarinsDad]

Until the late 80s when Voyager 2 passed Neptune, the then current scientific theory was that Neptune had very little in the way of winds due to its vast distance from the Sun and lack of solar heat for convection. However, Neptune has the fastest measured winds in the solar system, getting up over 2000 km per hour. Science was fairly advanced in the last 80s. Dark Matter theories had been around for 60 years. Einstein came up with most of his ideas 50 to 85 years earlier. How could scientists be so wrong on something so fundamental so recently?

The problem with Dark Energy (and Dark Matter) and some other theories where scientists are using a set of observations to come up with an educated guess, is that it's still pretty much a guess. Yes, these are educated and calculated guesses based on current data, but scientists really don't know yet. Until they do (requiring a lot more data, observation, and calculation), discussing theories like these is fun, but it's probably not going to lead to the actual truth (or at least when lay people such as myself discuss them on a web forum).

As an example, some scientists claim that the Large Hadron Collider can recreate conditions during the first few billionths of a second of the Big Bang. Well, maybe it can or maybe it cannot, but since nobody was around during those moments of the Big Bang, there's a chance that these scientists are wrong on this. They were wrong about Neptune. And, maybe scientists have found the Higgs Boson, but then again, even if they call it that, it might not actually be that.

Scientists have been searching for Dark Matter for nearly 80 years and still have not found it. Nor has any one set of Dark Matter equations actually worked for every galaxy that they used them on. Maybe part of the stellar attraction in galaxies is due to electromagnetic fields (which also influence at the square of the distance).

Dark Energy theory is only a little under 15 years old, so yeah, it's still fairly new. It might be wrong since it is primarily (but not solely) based on one type of observation and there is at least one alternative explanation, but it's still too early to know.

Interestly though, a lot of scientists think that the universal expansion is accelerating. Maybe it is. Maybe it isn't. But, we need other verification to be sure. If it isn't accelerating, then we get to throw out the most recent text books on at least part of the Lambda-CDM model.

The problem with many of our current theories of the universe are that many of them are built upon earlier theories that aren't necessarily 100% reliable or factual. Knock out one of the earlier ones and the current ones might collapse. For example, prove that the Big Bang never happened and many billions of dollars and lifetimes of research pretty much fall by the wayside.

 

[Umbran]

Dark Matter theories had been around for 60 years. Einstein came up with most of his ideas 50 to 85 years earlier. How could scientists be so wrong on something so fundamental so recently?

Well, at the moment we are talking more about dark energy than dark matter.

For dark energy: Einstein originally put in his cosmological constant, because he needed the term there in order to make the Universe static (neither expanding nor contracting). Edwin Hubble's observations proved that the universe was expanding - Einstein, given that information, felt the constant was unnecessary to explain the observed phenomenon, so he removed it.

It is only more recently that our instruments have become precise enough to measure the not only the expansion, but the acceleration of expansion. So, now with new data, we adjust again, and perhaps but Einstein's constant back in place.

The problem with Dark Energy (and Dark Matter) and some other theories where scientists are using a set of observations to come up with an educated guess, is that it's still pretty much a guess. Yes, these are educated and calculated guesses based on current data, but scientists really don't know yet.

The acceleration of expansion was first observed in 1998. In the years since, the fact that expansion is generally accelerating has been verified by different groups, using a wide variety of techniques. That the expansion of the visible universe is accelerating is pretty well observed, and not really guesswork at this point.

Until they do (requiring a lot more data, observation, and calculation), discussing theories like these is fun, but it's probably not going to lead to the actual truth (or at least when lay people such as myself discuss them on a web forum).

Are folks on this forum going to have some remarkable insight that clears up some fundamental issues of physics? Probably not. Can some folks learn a bit about the current thoughts of the universe? Certainly.

As an example, some scientists claim that the Large Hadron Collider can recreate conditions during the first few billionths of a second of the Big Bang. Well, maybe it can or maybe it cannot, but since nobody was around during those moments of the Big Bang, there's a chance that these scientists are wrong on this.

So what?

There is "a chance" for just about anything. There's "a chance" that Morrus is actually Elvis Presley. We cannot allow "a chance" that we might be wrong to stop us from trying and thinking. The scientific method not only allows that there's a chance that we might be wrong, it absolutely requires we try anyway. The whole process of science is, in the end, a big trial-and-error game. Error is a major portion of the process, as error often reveals more information than being correct!

Beyond science, all of life is a process of taking the best shot with the information and tools at hand. There's a chance that the next job you take will be a really, really bad career move, in ways that you will not be able to see. Do you allow that to paralyze you?

They were wrong about Neptune. And, maybe scientists have found the Higgs Boson, but then again, even if they call it that, it might not actually be that.

If you read the actual writing of the scientists involved in those experiments, you'll find that they didn't say they found the Higgs. They said they found a candidate for the Higgs. It has the right mass. It decays in some of the right ways. They'd like to find some of it's other decay modes, and verify its parity, before really claiming it to be a Standard Model Higgs.

Scientists have been searching for Dark Matter for nearly 80 years and still have not found it.

How long you've been looking isn't really a measure though. Mankind was trying to fly for millennia (since Daedalus, at least), and it took the Wright Brothers to get us off the ground.

Interestly though, a lot of scientists think that the universal expansion is accelerating. Maybe it is. Maybe it isn't. But, we need other verification to be sure. If it isn't accelerating, then we get to throw out the most recent text books on at least part of the Lambda-CDM model.

See above - the acceleration of expansion in our visible universe is pretty well observed and accepted at this point. The basic way out of invoking something exotic like Dark Energy is to have this be only a local phenomenon. Historically, though, theories that have us living in a particularly special part of the universe have tended to be wrong.

For example, prove that the Big Bang never happened and many billions of dollars and lifetimes of research pretty much fall by the wayside.

Yes. And? So what? Is there some preferable alternative?