Why space expansion affects matter?

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Solution 1

• Space actually has energy, vacuum energy. It has been shown by various experiments and can be explained by Quantum Mechanics. So more space means more energy. Although it probably can't be used to do work, it does act to increase expansion. Check out the wikipedia page for more info.

• Although galaxies are massive, they are far away and thus the resulting acceleration towards each other is weak. If the space between galaxies is expanding at a faster rate than their mutual atraction; galaxies will move apart.

Imagine a football player running from one endzone toward the other as fast as he could. However the distance between them endzones doubles every 4 seconds. The endzones are not moving; the space between them is growing. There is no way the player could hope to reach his goal. In a short time he would not be able to see the other end.

The scary thing is this not only goes for the field but everything. The player himself would be ripped apart as his own body parts moved farther and farther apart. If space was expanding fast enough the attractive forces keeping black holes or even atoms together would not be enough. This what is commonly referred to as the Big Rip.

Luckily the expansion is slow enough that only "distant" galaxies are moving away from each other. Gravitational forces in a solar system or a galaxy are more than enough to withstand expansion. The larger the scale of the system the more expansion has to play.

• When we talk about expansion we are not talking about objects moving away because of their great velocities against a static backdrop.

What you're describing is like marbles on a grid. The marbles get farther away due to their velocities relative to the grid pointing in opposite directions.

Instead expansion is like the grid growing in scale. This effects the distances between the marbles independently of their velocities; which is why we observe all far away objects as moving away from us. If it wasn't for expansion we would expect a more random distribution. It is only at smaller scales where expansion is less of a factor that we can see objects moving toward us such as Andromeda's Galaxy.

We are not in any sense moving away from the center of the universe. The idea of the Big Bang and Inflation is that not only every thing, matter and energy; but everywhere was contained in the Big Bang.

Space is "nothing"

Not really. General relativity tells us that space-time is an actual living entity that responds to what's matter doing and matter responds to the way space-time is curved. Or as John Wheeler put it: "matter tells Spacetime how to curve, and Spacetime tells matter how to move." For an introduction into general relativity, you can start at this wikipedia article.

Gravity doesn't have a cutoff distance

You are talking just classical Newtonian gravity. But this isn't how gravity works on big scales. On big scales gravitation is just an appearance caused by the fact that the space-time is curved. If it is curved in a certain way then the appearance is that the objects attract each other. Like this:

But on huge scales these local, attractive, deformations are completely negligible. When you take whole space-time into account, it is curved differently (more onto this in the last section). And the important fact is that the space-time expands (like when you blow into a balloon). Because it expands, the distances between any two points are getting bigger and it seems like everything is getting away from you. So that gravitation is actually a repulsive force on big scales (or at least, it can be; again see the last section for a clarification).

Wouldn't the expansion decelerate?

There are many solutions of Eistein's equations. Some of them are stationary, some of them are expanding (either at accelerated rate or decelerated rate), some of them revert from expansion to retraction. Now, which of these solutions is correct depends on the precise conditions at (or more precisely shortly after) the big bang.

Important part of the present theoretical understanding of the space-time expansion is that the cosmological constant is non-zero and this is what accelerates the expansion. The complete picture about the matter content of our universe and its relation to the expansion of the universe is called Lambda-CDM (Lambda is an alias for the cosmological constant and CDM is a cold dark matter, which makes up most of the matter in the universe). Now, cosmological constant is also called dark energy. But currently nothing much is known about these matters and major conceptual problems in modern theoretical physics and cosmology have to do with the nature of the cosmological constant.

Solution 3

I actually think you've been misled. Usually (in my experience) when people say that space is expanding, what they mean is that the universe is expanding - but that just means that the objects in the universe are getting further apart (or, in the case of a continuous mass/energy distribution, that the density is decreasing with time). This is what the cosmological scale factor $a(\tau)$ is for: it describes the change in the distance between two objects (like galaxies) whose motion is subject to large-scale interactions only.

$$r(t_1) = \frac{a(t_1)}{a(t_2)}r(t_2)$$

Now, because $a(\tau)$ is part of the metric, which specifies the distortion (or "curvature") of spacetime, it's easy to think that this scale factor would characterize the expansion of space itself. But I think that's the wrong interpretation, or at least a confusing interpretation. The scale factor really just relates to measurable distances between objects. So instead of trying to figure out what it means for space to expand, just think about things getting further apart.

Moving on, you're right to say that, if things behave the way we would intuitively expect them to, the expansion of the universe should be slowing down due to gravitational attraction. But the best experimental observations that I am aware of show that the opposite is true: the expansion is speeding up. Unless you're prepared to argue that the experiments were performed or interpreted incorrectly, that means that something nonintuitive must be going on.

There's still a lot of debate about what exactly could be making the expansion of the universe speed up. Whatever it is, cosmologists are calling it dark energy (back in Einstein's day they called it the "cosmological constant"), but nobody has a satisfactory explanation of why this dark energy exists, or what its properties might be, other than the fact that it makes the universe's expansion speed up. This is one of the biggest open questions in modern cosmology.

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serg

Open source projects: Chrome extension that adds tab formatting features to all textareas (view others) Java HTML compressor/minifier jQuery plugin to put a spinner over loading elements (loading mask) Recent hobby projects: Xbox gamerpics preview Cover videos of popular songs. Currently abandoned :( Would love to find someone who would be interested in helping me with it. You can contact me at serg472 at gmail.com.

Updated on July 12, 2020

• serg over 3 years

If space itself is expanding, then why would it have any effect on matter (separates distant galaxies)?

• Space is "nothing", and if "nothing" becomes bigger "nothing" it's still a "nothing" that shouldn't interact with matter in any way (it doesn't have mass, energy, etc).

• Gravity doesn't have a cutoff distance afaik, so even the most distant galaxies should be attracted to each other. Gravity force would be very very tiny, but it would still dominate "nothing" from space expansion.

• Lets take inertia from Big Bang into account. Inertia would be the primary force that moves galaxies away comparing to their tiny gravity and even more tiny, if any, force of our "nothing" that is still expanding inbetween. Wouldn't expansion decelerate if driven mostly by inertia?

• Malabarba almost 13 years
Inertia would be the primary force. I can tell you know what you're talking about, and you don't mean Newtonian Force in that sentence, but I suggest you change force to something like factor because new students really tend to think inertia is actually a force.
• serg almost 13 years
So you mean objects are just flying apart under unknown force in a static space, not because space itself is expanding and pulling objects apart? I always was under impression that fabric of space is being created, so objects move apart because more "emptiness" being created between objects, while objects don't actually move.
• Marek almost 13 years
@serg: It's just as you say. "Space is being created", or rather, "space-time expands" and the fact that things move apart is just an illusion. So I don't agree with David that it's wrong to think about expansion of space-time in terms of $a(\tau)$. According to GR that is precisely what happens. E.g. if our universe were a 3-sphere then you could (in principle) actually measure it's diameter along main circle and this would indeed be increasing with time.
• David Z almost 13 years
@Marek: I think you misinterpreted my answer. I wasn't saying that it's wrong to think about expansion of spacetime in terms of $a(\tau)$, but rather that it's misleading to think about $a(\tau)$ as characterizing space itself. I simply meant that it's better to associate it with measurable distances, not some abstract notion of "space." If the universe were a 3-sphere, its circumference would be one of those measurable distances; there's certainly no reason you couldn't associate it with $a(\tau)$.
• David Z almost 13 years
(cont) And I have to disagree with your comment in one point: I would say things are moving apart, and the idea that "space is being created" is nonsense - unless you develop a precise definition for what it means to create space.
• David Z almost 13 years
@serg: It seems like you have this idea that space is some entity separate from matter and energy, i.e. that it can expand while the objects in it stay still or it can stay static while the objects in it move apart. Do your best to forget that. The whole "fabric of spacetime" analogy is horribly misleading if you push it beyond what it was meant to explain, as I think you're doing here.
• Jerry Schirmer almost 13 years
Describing these things in terms of words is tricky. Take the flat Roberston-Walker metric, and make the coordinate transformation $R=a(\tau)r$. Then the metric becomes $ds^{2}=-\left(1-\left(\frac{\dot a R}{a}\right)^{2}\right)d\tau^{2} + 2d\tau dR \left(\frac{dot a R}{a}\right)+dR^{2}+R^{2}d\Omega^{2}$. Now, it literally appears that there is a frame dragging effect (the $g_{\tau R}$ term) describing the expansion of space, and that distances are fixed. Which formulation is right? Both of them. It's a matter of interpretation.
• Marek almost 13 years
@David: ah, all right then. But if I remember FRLW metric correctly, $a(\tau)$ has a very clear connection to the actual size of the universe (at least for the compact case).
• Marek almost 13 years
@David: it depends. Both statements "they are moving apart" and "they are not moving apart" are correct depending on how you interpret them. It's true that there is an effective motion arising from the expansion but this doesn't come from the movement of the objects themselves.
• Marek almost 13 years
@David: why do you have problems with "space is being created"? Sure, it's just popular view but it's totally true. Take again the 3-sphere. It has some volume. You can measure that. This volume is increasing in time. So indeed, space must've been created.
• David Z almost 13 years
@Marek: well, there you have a precise definition of what it means to create space: increase the circumference (and thus the volume) of the universe. That's perfectly fine. What I really have a problem with is trying to hold a technical discussion using words whose meanings are unclear. Also re: 2 comments up, how do you define the movement of the objects themselves, except by the change in their relative position?
• Marek almost 13 years
@David: I am not trying to hold a technical discussion. I thought we are trying to explain something to a layman ;-)
• Marek almost 13 years
@David: regarding movement... do you actually know how to derive FRLW? You start with stationary observers (which you can do thanks to homogeneity assumption) that are stationary w.r.t. the fluid that fills the space. So yes, you can define absolute movement for these kinds (of very special) space-times.
• David Z almost 13 years
@Marek: well, in the answers I suppose we were trying to explain to laymen, but now we are in the middle of a technical discussion ;-) Anyway, I think this whole discussion results from opposing definitions. I'm defining "movement" to be a change in measured relative position, whereas if I now understand you correctly, you're defining it as something more like a change of comoving coordinate - which is fine, of course; sorry for making a fuss about it! I just think that the latter is likely to be much more confusing to someone who isn't familiar with GR at the mathematical level.
• Marek almost 13 years
@David: fair enough. Regarding movement: precisely. There are two independent definitions and none of them is more correct. But I think @serg had a pretty good picture of this in mind (that I also mentioned) already. Namely, that you can imagine the universe as a balloon. As you blow in it it gets bigger and distances increase but the actual rubber molecules aren't really getting anywhere (in the second sense of movement). Just intermolecular distances increase (which corresponds to the expanding space and decreasing density of the galactic fluid).
• Gerard almost 13 years
J. Wheeler's phrase: 'matter tells spacetime how to curve, and spacetime tells matter how to move' implies that matter tells itself how to move - now what should we make thereof?
• Marek almost 13 years
@Gerard: what about it? It should be completely natural. Just take Newtonian gravity: one piece of matter (Earth) tells another piece of matter (Moon) how to move. GR just adds a mediator of that action.
• Jerry Schirmer almost 13 years
@David @Marek: That's why my attitude about these things is generally "calculate the relevant physical property using the rules of differential geometry and GR" and only then, provide a plain english set of words to interpret the result. Otherwise, you just end up talking in circles about things that might be equivalent in different coordinates. Proper distances between galaxies increase in a FRLW spacetime. <b>Why</b> this happens depends on what coordinate system/interpretation scheme you're using, and different schemes clarify some things and obscure others.
• David Z almost 13 years
@Jerry: well said. I agree with that too (that the real results come from the calculations), I guess it's just a little too easy to get carried away arguing about the interpretations.
• Gerard almost 13 years
Of course, nothing new. When GR introduces a mediator, maybe it can be compared to the 'ether' concept. That was a mediator too, and after discarding it in SR a new mediator was introduced in GR?
• Marek almost 13 years
@Gerard: yes, in a sense you could think of the space-time as a kind of 'ether'. Although it's much more sophisticated that the naive concept of ether people proposed before SR.
• Marek almost 13 years
@Jerry Schirmer: well said indeed.
• David Z almost 13 years
In your last bullet point, I think you should specify that by "velocity" (of the marbles) you are referring to movement relative to the grid. (One could also define velocity as being the change in relative position, and in that sense the marbles would have a velocity apart from each other even if they are not moving relative to the grid)
• David almost 13 years
@Zaslavsky How would you suggest I rephrase the statement to make it more apparent? I would like to keep it sort as possible but I see how it can be misleading. I tried to improve it. I think it could be phrased better.
• David Z almost 13 years
Well, the edit you made seems fine. I guess you could also have said "...velocities through space itself" or something like that. I can't think of a particularly great way to phrase it off the top of my head.
• sammy gerbil almost 4 years
The football field analogy doesn't make (common) sense. How can the space between the endzones expand without the endzones moving?