/sci/ - Science & Math

I..

uh

...

I'm curious too

Well, to start, if two masses are at a fixed distance, then by definition the system isn't expanding.

They would lose gravitational attraction over time if the gravity isn't strong enough to resist the expansion.

So in order to remain at a fixed distance, they have to move closer to eachother? Thats the term for this point? Only close enough for gravity to pull enough to overcome expansion? Some bastard cousin of orbit?

Also, what of expansion speeding up?

To say a bit more, the question is probably posited on the idea that there's some mysterious force causing the universe to expand. But that's not what's happening. The galaxies are moving apart, and with neither forces nor gravitation, they would continue to move apart at the same rate indefinitely. Of course, in the real world, there is a mysterious source of gravitation which causes the galaxies to move apart faster.

maybe it does.
you're a scientist prove it

As for the "dark energy" which causes expansion to accelerate, it will have a small effect on two masses close to each other. But very small. Dark energy is very sparse. There's even less matter in the universe, but matter comes in clumps. Within a galaxy, there's a lot more ordinary matter and dark matter than there is dark energy.

>>1570953
>Of course, in the real world, there is a mysterious source of gravitation which causes the galaxies to move apart faster.

> Of course, in the real world, there might be an additional form of energy which causes the universe to accelerate its expansion.

fix'd, but still not happy with "form of energy"

>>1570870
two objects at a fixed distance will radiate energy, and loose mass. Thermodynamics.

Wouldn't galaxies simply physically moving away from eachother at an accelerating rate imply an impartation of momentum?

Maybe the universe isn't expanding, maybe everything inside of it is shrinking.

>>1571010
Are you saying thats connected to expansion? I'm not being snide, Im seriously interested.

I assume you're referring to planets. Planets orbit each other. Aligned one way the planets will be pulled apart by universe expansion. Aligned the opposite way the planets will be pulled together by universe expansion. If you're not talking about planets i don't know what you're talking about.

>>1571023
Why do you specify planets?

>>1571027
it might be
http://en.wikipedia.org/wiki/Gravity_as_an_entropic_force

>>1571033

Because they are the most common masses that exhibited obvious gravitational effects.

>>1571026
Oops
>>1571037 was a reply to you

>>1571039
>they are the most common masses

what about stars?

>>1571023
Seriously.

>>1571067
Count the stars in this solar system, then count the planets.

>>1571073
http://www.atlasoftheuniverse.com/

What, are you 12???

>>1571010
you'redoingitwrong

So we're back to expansion being physical movement, rather than some kind of general spacetime warp?

>>1571075
Nothing to explain, >>1571023 is wrong.

>>1571087
maybe it's a bit of both

>>1571087
Spacetime curvature, a.k.a. gravitation, causes changes in the rate at which the universe expands.

>>1571087
No, the entire spacetime is expanding, together with the clusters of matter getting farther and farther away from each other.

0=planet A
X= planet B
-=Gravity
$= center of the universe

Time 1
0----X---------$
Time 2
$---------0----X

Expansion of the universe cancels out

>>1570870
The expansion of the universe is uniform, but this implies that the rate of expansion at small scales is barely noticeable.
Even at the scale of our Galaxy the expansion is still small, let alone at the scale of our Solar System, or ourselves.

Clusters of galaxies can be bound together just enough to *locally* overcome the expansion, so they will keep together. However, at larger scales the expansion dominates and is able to pull things apart.

Back again with galactic clusters, this also means that there are certain galaxies in the very outskirts of them that are "almost in equilibrium", balanced by gravity and expansion!

Picture related, it's a cluster of galaxies.

>>1571107
Oh, I see who you are. Take your crackpot "theories" elsewhere.

OP, disregard these posts:
>>1571107
>>1571023
There is not even a center of the universe.

http://map.gsfc.nasa.gov/site/faq.html
http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf

Im getting conflicting information here. It feels like some people are saying expansion is a property of spacetime, and other people are saying its just galaxies moving around.

If there no matter in the universe, only energy, would that energy still be redshifted due to expansion?

>>1571125
The second link should be:
http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_BigBang.pdf

The other one is too advanced.

>>1571129
Energy is not a physical entity. It is the capacity of a physical system to do mechanical work onto another system.
With no matter, if you have only photons (assuming that's possible), yes, they would be redshifted by expansion.

The point is that those objects that are far enough from each other, so they are affected sufficiently by the expansion, will indeed experience a change in gravitational attraction. If they are close enough or binded gravitationally or chemically, it would take expansion an *incredibly long* (and I really mean long) time to separate them.

>>1571129
More clearly: expansion is something that happens in our universe to spacetime itself. As a consequence, it drags everything in it apart, but the relative expansion between any two points depends on the "distance" (that word will be enough for now, but it's a bit more complicated) between them. Points very far apart measure larger respective expansion speeds than closer pairs of points.
The gravitational attraction diminishes in time, but for small scales like our galaxy or even a small local group of galaxies, is not so great. In our Solar System and smaller scales the expansion is so small that we can forget about it.

>>1571157
>With no matter, if you have only photons (assuming that's possible), yes, they would be redshifted by expansion.
In order to say that light has been redshifted, you need an initial reference frame in which to measure its initial frequency. You don't need matter (the cosmic microwave background, for example, will do), but you do need something to establish this frame.

>>1571180
Sure, but that's for the sake of a measurement for confirmation. The exercise is a thought experiment.

dearest OP, may i enquire as to the cource of the picture - the artist? thanks

>>1571184
Real experiment or a thought experiment, frequency depends on the reference frame.

>>1571185
not OP, but look in the upper right corner
http://www.redbubble.com/people/surrealismart/art/1588737-1-arrested-expansion-or-cardiac-arrest

>>1571129
>some people are saying expansion is a property of spacetime, and other people are saying its just galaxies moving around.

theres a spacetime "structure" to that moving around. It's not just random

http://en.wikipedia.org/wiki/Hubble%27s_law

>>1571189
Ok, suppose I am a massless ghost travelling such that I WOULD observe the light at the same frequency that it originally existed. My question is if the light would be shifted by the time it reaches me, because of expansion.

>>1571192
"The most common cause of cardiac arrest is a heart rhythm disorder or arrhythmia called ventricular fibrillation (VF). The heart has a built-in electrical system. When a contraction happens, blood is pumped. But in ventricular fibrillation, the electrical signals that control the pumping of the heart suddenly become rapid and chaotic. As a result, the lower chambers of the heart, the ventricles, begin to quiver (fibrillate) instead of contract, and they can no longer pump blood from the heart to the rest of the body. If blood cannot flow to the brain, it becomes starved of oxygen, and the person loses consciousness in seconds."

I do not understand art.

>>1571261
There's no such thing as "the same frequency that it originally existed." Only "the frequency that it originally existed" in the reference frame of the galaxy in which it was emitted.

Without matter, it doesn't even make sense to ask if the universe is expanding.

>>1571281
A photon with frequency x is emitted. I am some distance away, with zero momentum. I should receive the photon after a time and calculate its frequency to be x. Or will its frequency be lower than x, redshifted, due to the expansion of spacetime that it travelled through to reach me?

>>1571315
With qualifiers added to make the statements meaningful:

In some distant galaxy, a photon with frequency x in the reference frame of that galaxy is emitted. You are some distance away, with zero momentum in the reference frame of the piece of ground you are standing on. You receive the photon, and its frequency is lower than x, redshifted, due to the expansion of the universe.