Dark Energy Question

[Ser Scot A Ellison]

Okay i was just watching the episode of "Cosmos" (Tyson version) that discusses "Dark Energy".  It discusses the hypothesis regarding Dark Energy and how Dark Energy is increasing the speed with which the Universe expands.  It made me thnk about the fact that space itself, based on "inflation theory", can expand faster than the speed of light.  How do we know that the accleration of the expansion of the Universe isn't just an expression of the expansion of Space itself like the early rapid expansion?  In other words is thinking about this as galaxies moving faster and faster through space completely wrong?  What if this acceleration is a reflection of Space itself continuing to "inflate"?  

Can we use observation to determine whether the acceleration is of Space expanding or of Galaxies getting faster moving through space?  Would such a distinction matter?

Thanks.

[Starkess]

I'm not entirely sure I follow what you're asking regarding inflation vs dark energy, but the expansion of the universe is due to acceleration of space and not just galaxies moving through space. This ties back to the idea that there is no "center" of the universe--you would see the universe expanding outwards from any galaxy, not just this one. Some analogies that are commonly used are raisins expanding away from one another in rising bread or dots on the surface of a balloon moving away from one another as the balloon expands.  

[maarsen]

Dark energy has been invoked to explain to explain the expansion of the universe. There is a problem with dark energy as it is undefined and is really not much of an explanation for anything. Physics is in a bind for the last few years and dark matter and dark energy have been invoked as explanations with no real evidence or theoretical backing. 

If you really want to grab onto a theory of physics, the flavour of the month is loop quantum gravity.

[Ser Scot A Ellison]

10 hours ago, Starkess said:

I'm not entirely sure I follow what you're asking regarding inflation vs dark energy, but the expansion of the universe is due to acceleration of space and not just galaxies moving through space. This ties back to the idea that there is no "center" of the universe--you would see the universe expanding outwards from any galaxy, not just this one. Some analogies that are commonly used are raisins expanding away from one another in rising bread or dots on the surface of a balloon moving away from one another as the balloon expands.  

Actually... even that works with inflation.  As more "space" comes to be it would reduce the effects of gravity because it would increase the distance between warps in spacetime.  The fact that a few raisins get closer to each othet doesn't mean that the overall inflation isn't happening.  

[my sincere apologies I'm not trying to mansplain here and I can see how it sounds like that I am.  I'm trying, unsuccessfully, to get a better handle on what I'm thinking about]

[Kalbear]

Just now, Ser Scot A Ellison said:

Actually... even that works with inflation.  As more "space" comes to be it would reduce the effects of gravity because it would increase the distance between warps in spacetime.  The fact that a few raisins get closer to each othet doesn't mean that the overall inflation isn't happening.  

The problem isn't that it's being counteracted - it's that the relative rate of the expansion of the universe (as far as we can tell) is increasing. If something isn't causing things to be pushed apart, we would expect gravity to diminish but still pull things together. 

And that's not happening.

And we don't know why. 

[Ser Scot A Ellison]

4 minutes ago, Kalbear said:

The problem isn't that it's being counteracted - it's that the relative rate of the expansion of the universe (as far as we can tell) is increasing. If something isn't causing things to be pushed apart, we would expect gravity to diminish but still pull things together. 

And that's not happening.

And we don't know why. 

But if inflation is weaking the gravitational attraction between matter that warps spacetime shouldn't gravity be getting weaker as most warps get more distant from each other?

[Starkess]

7 minutes ago, Ser Scot A Ellison said:

Actually... even that works with inflation.  As more "space" comes to be it would reduce the effects of gravity because it would increase the distance between warps in spacetime.  The fact that a few raisins get closer to each othet doesn't mean that the overall inflation isn't happening.  

Yes, the expansion is only seen on very large scales. Systems that are bound gravitationally (ranging from something as small as the solar system to something as large as a galaxy cluster) aren't expanding internally.

[Kalbear]

Just now, Ser Scot A Ellison said:

But if inflation is weaking the gravitational attraction between matter that warps spacetime shouldn't gravity be getting weaker as most warps get more distant from each other?

Sure! But that's not the point. Even if gravity is getting weaker, we would still see a consistent acceleration towards the center...of something. Instead, we see expansion - and not only do we see expansion, we see acceleration of expansion. This implies some kind of force is pushing things out (or repelling, or whatever), and that is not explained solely by inverse square law dynamics of gravity. 

[Sivin]

Now I'm confused. If the universe's expansion was accelerating wouldn't stuff like... get smooshed?

[Kalbear]

Just now, WinterFox said:

Now I'm confused. If the universe's expansion was accelerating wouldn't stuff like... get smooshed?

I don't know why. You're on a globe under constant acceleration and facing centripetal force. Do you get smooshed? 

[Sivin]

2 hours ago, Kalbear said:

I don't know why. You're on a globe under constant acceleration and facing centripetal force. Do you get smooshed? 

But that's velocity, not acceleration 

[Kalbear]

2 minutes ago, WinterFox said:

But that's velocity, not acceleration 

No, both of those are acceleration. Gravity accelerates you constantly at...well, 1 gravity. Centripetal force is a force as well, and a continuous acceleration. 

[Altherion]

1 hour ago, Ser Scot A Ellison said:

But if inflation is weaking the gravitational attraction between matter that warps spacetime shouldn't gravity be getting weaker as most warps get more distant from each other?

You are using the terminology in a rather confusing way. Inflation customarily refers to the extremely rapid expansion of space during a brief period of time very near the beginning of the universe. It's something that happened once and then stopped. I suspect what you mean is that space is expanding and this expansion is accelerating, but this is not at all the same thing as inflation: it is much, much slower, it acts on a different scale and its cause is almost certainly not the same.

Gravity is a fundamental interaction resulting from the curvature of spacetime around concentrations of energy (not just matter -- massless particles can also participate). As far as we can tell, gravity itself is not getting stronger or weaker. What you almost certainly mean is that its influence diminish as galaxy clusters move away from each other and this is true, but the strength of the interaction generally does not include the characteristic distance between entities subject to that interaction.

Starkess gave you a good answer to your original question. If you want a more detailed answer which includes the most commonly accepted idea for why the expansion of space is accelerating (i.e. what is "dark energy", where its anti-gravitational effect comes from and how do we know this), I would suggest watching this SpaceTime YouTube playlist. Unfortunately, there is no intuitive explanation and a lot of things that we usually take for granted aren't valid anymore (e.g. conservation of energy). You really have to dig into GR to understand it and those videos do it about as painlessly as possible.

If you're short on time, you can skip 4 and 6 on that list. 4 is an interesting question which you may wish to figure out for yourself, but the contest for it has expired. 6 is an illustration of why one should be wary of revolutionary results reported in the media, but it mostly tells you about the scientific process rather than dark energy itself.

[Loge]

5 hours ago, maarsen said:

Dark energy has been invoked to explain to explain the expansion of the universe. There is a problem with dark energy as it is undefined and is really not much of an explanation for anything. Physics is in a bind for the last few years and dark matter and dark energy have been invoked as explanations with no real evidence or theoretical backing. 

If you really want to grab onto a theory of physics, the flavour of the month is loop quantum gravity.

Actually, what's called Dark Energy nowadays was already in Einstein's field equations of General Relativity. But back then people believed in a stationary universe, so Einstein dismissed it. 

As for Dark Matter, that has been postulated to explain the orbital motions of stars in galaxies. Newton's law of gravity allows us to derive the mass of a star from the distance of a planet orbiting the star and the orbital period. Likewise, it allows us to derive the mass distribution in a galaxy from its rotation curve, i.e. how the orbital periods of the stars orbiting the galactic center vary with increasing distance from that center. Except, things don't quite work out there. Stars orbit the galactic center faster than they should. That means either Newton's theory of gravity doesn't hold over long distances or there's a lot more stuff out there than we see. As far as I know non-Newtonian gravity causes some problem of its own, but I'm no expert so I can't tell you any details. Most astronomers favor the other explanation, namely that there's stuff out there that we don't see. The problem with that is that there has to be a lot of it and all the matter we know (baryonic matter) is made of electrically charged particles and therefore emits electromagnetic radiation. The invisible ("dark") stuff would therefore have to be some so far unknown typeof matter. Particle physics should eventually be able to either find it or prove its non-existance, but we aren't there yet.

[Sivin]

2 hours ago, Kalbear said:

No, both of those are acceleration. Gravity accelerates you constantly at...well, 1 gravity. Centripetal force is a force as well, and a continuous acceleration. 

Now I'm seriously confused. Sorry, I think physics is wicked cool but I failed it hardcore in school. I thought that acceleration (in plebeian terms) is a force that you can actively feel pressing 'against' you, while velocity is a constant and 'adjusted' measure of distance traversed in a given time.

My understanding to so far has been that I know Sol is hurtling through space at a titanic speed while the earth is circling the sun at millions of miles per hour and the planet spins at roughly a thousand miles per hour, but that we're actively accelerating isn't quite registering to me.

I mean, I thought that 1g is a measurement of how forcefully the earth pulls against us on its surface. But if we're accelerating towards or from something, wouldn't that effect be measurable?

Like I said, I failed Physics in high school, I'll take your word for it Bear. If you can give me some kind of links to educational material (don't feel obligated on my behalf) I would be very grateful.

[Maltaran]

Acceleration and velocity are both vector terms though, which means the direction is important. If you are travelling in one direction at 30mph, and then turn 180 degrees and travel in the other direction at 30mph, your speed may be the same but your velocity has changed by 60mph.

[Loge]

You have to accelerate to not feel gravity. You feel the earth's gravity because you don't accelerate. Astronauts in orbit don't feel the earth's gravity because they constantly accelerate towards the earths  center of mass. 

[Ser Scot A Ellison]

Altherion,

I understand that "inflation" is a phenomenon that occurred once in the early Universe.  What I wonder is whether it is also a continuing phenomenon occurring in the background that could be used to explain the acceleration of the spread of galactic clusters?  

Doesthat make more sense?  In other words some believe "inflation" happened in a big way early in the history of the Universe, then stopped.  What if it didn't stop but merely slowed down a lot?  What if that small but constant inflation can account for the acceleration?

[Xray the Enforcer]

10 hours ago, Ser Scot A Ellison said:

Actually... even that works with inflation.  As more "space" comes to be it would reduce the effects of gravity because it would increase the distance between warps in spacetime.  The fact that a few raisins get closer to each othet doesn't mean that the overall inflation isn't happening.  

did you just "um actually" someone who is about to embark on a PhD in physics? Seriously? #menexplainthingstome

[Ser Scot A Ellison]

3 minutes ago, Xray the Enforcer said:

did you just "um actually" someone who is about to embark on a PhD in physics? Seriously? #menexplainthingstome

Probably.  But I'd say that to anyone.  I'm not claiming to be perfectly correct.  Just that I think that works as I'm thinking about this idea.  Which is only an idea.  If I'm wrong... that's cool.

I wrote it not as a claim of superiority but because that's how I talk about stuff like this in person.