# Gravity and Universal expansion

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1. Apr 29, 2015

### Ahmed Nabi

Hi everyone!
I am new to cosmology and have come across this unbelieving interesting field quite recently ( after reading 'A Brief History of Time' to be honest ). This is my first post in PF.

Hubble first discover that the universe is expanding with non-constant, accelerating velocity. Also according to GR, gravity is caused by curvature of space-time caused by Earth's Mass. My question is, if earth is (being a part of the expanding universe) expanding with accelerating velocity in a particular direction, where does the counter gravity produced by it goes? For simplicity, if we consider the elevator as earth we can feel extra 'push' to floor while elevator is going upward. Where does that extra push goes in case of earth?

My question might seem a bit awkward to someone may be but I believe in 'no shame in asking question' principle :)

2. Apr 29, 2015

### Orodruin

Staff Emeritus
Hubble never discovered that the expansion was accelerating. This is a recent discovery.

No, gravity is curvature of space-time created by all energy and momentum content in the Universe.

This is a misconception. The Earth is not accelerating in any direction. The expansion of the universe is an expansion rate, which means that distances are getting larger at a speed which is directly proportional to the distance itself. It does not have a direction associated to it.

3. Apr 29, 2015

### Bandersnatch

Hi Ahmed, welcome to PF!

Let me expand on Orodruin's answer.

The first thing I'd tell you, is that cosmology in popular treatments is rife with imprecise language that can easily confuse the reader if he's not especially careful. That is not entirely a fault of the authors/speakers, as this highly abstract and mathematics-heavy field is hard, if not impossible, to describe unambiguously in words of everyday communication that shies away from equations and graphs.

Already I can see a number of misconceptions you've picked up. For example:
It is not correct to say that the universe is expanding at a velocity, be it constant or not. The universe is expanding at a rate. It grows by a certain percentage over time. By analogy, it's equally incorrect to say it's expanding at velocity as it is to say that deposits on a savings account in a bank grow by a certain amount of \$ (or whatever) per month - savings grow by some percentage per month. The exact amount varies depending on how much money you've deposited, and how long they had been there, but in all cases the percentage growth rate is the same.
So, in concrete terms, our universe is growing (currently) at about 1/144th of a percent per million years. That's how it should be understood - as a percentage growth, not a velocity (1/144th of a percent per million years is another way of writing the Hubble constant, by the way).

Hubble was first to notice that there is such a rate of growth. He noticed that a galaxy twice as far would recede twice as fast. In the same way as twice as much money in a bank will grow by twice the amount as it otherwise would. The discovery of importance here is not the amount of recession/growth (specific to any one single galaxy only), but the overall rate of recession that can be deduced from those individual measurements.
The famous observation is represented by this graph:

where the circles are individual galaxies (nebulae, as they were classified back then).
The general trend of velocities increasing the farther away the galaxy is can be observed.

As Orodruin mentioned, the changing rate of growth is a recent (the last two decades) discovery.

Another misconception is that there is any directionality to the expansion. This is a natural conclusion when one imagines the universe as expanding from some concrete centre as if it were a big explosion of the kind we observe on Earth.

The universal expansion is indeed universal - it applies to every distance everywhere equally (same percentage growth rate), and does not have a centre. The big bang is not to be understood as an explosion seeding the empty space with matter, but as an already existing dense state of (possibly infinite) universe undergoing expansion, diluting and cooling. It did not happen at some point in space - it happened and still happens everywhere.

As such, all observers, regardless of where they are, see themselves as stationary, and all (far away) galaxies as receding from them. This is what the often-invoked balloon analogy tries to aid with visualising. If you pick any point on an expanding balloon's surface to be an observer, it'll measure every other point on the balloon as receding from it with velocity increasing with distance.

So, each point in the expanding universe is effectively stationary. As such, there is no associated acceleration imitating gravity, of the kind you'd experience in an elevator.

Finally, gravitationally-bound objects do not expand together with the rest of space, although I don't think you actually meant that (rather, you meant that Earth is a part of the expansion, being carried by it from some central point - which was addressed above).
You need to go beyond the scale of clusters of galaxies to observe the expansion.
This can be in a simplified way understood in terms of the escape velocity - from Hubble's graph (and Hubble Law generally), you can find the recession velocity of points some distance apart. The escape velocity is determined by all the mass contained in the radius equal to that distance. If the recession velocity is lower than the escape velocity thus calculated, the mass within that radius will remain gravitationally bound, and resist the expansion.
As mentioned before, the recession velocity becomes great enough only on large cosmological scales.

There are a lot of good materials to read on the subject available online, with varying level of complexity. One of the best, clearest, layman-oriented explanations of the current views on cosmology comes from a Scientific American article specifically addressing the most common misconceptions arising when first encountering the balloon analogy (but it has a broader appeal than just that). To be found here:
http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

We can direct you to more sources should you need them. Spending some time browsing the cosmology section of the forum, where similar questions have been often asked and responded to, might be also a good idea.

4. Apr 29, 2015

### Chalnoth

There is no counter "push". The accelerated expansion is caused by gravity. If you are free-falling in a gravitational field, you feel weightless. Think of the astronauts in orbit: they are in a gravitational field, and are accelerating (the circular motion around the Earth requires constant acceleration towards the Earth). But they are weightless. So no matter the impact of dark energy, we could not feel it in the way we feel an elevator starting and stopping.

5. Apr 29, 2015

### Ahmed Nabi

Thank you @Orodruin, @Bandersnatch and @Chalnoth for your replies. I guess I have adopted a lot of misconception at the very start on which I have to work on.
This makes it pretty much clear. Thank you again

6. Apr 30, 2015

### phinds

7. Apr 30, 2015

### Chalnoth

By the way, for the specific way in which the cosmological constant interacts with gravity, here is the Newtonian approximation of the gravitational force on a mass $m$ by a mass $M$:

$$F = {-GmM \over r^2} + {\Lambda m r \over 3}$$

So this is a little weird. Here we have an attractive force between any two masses which is proportional to each mass, but also a small outward acceleration that is only proportional to the mass being acted on. What this means, essentially, is that the cosmological constant creates a little repulsive acceleration between any two objects.

Now, the value of this outward push from the cosmological constant is exceedingly tiny until you get to cosmological scales.

For example, the Sun pulls on the Earth with a force of about $10^{22}N$. The little push on the Earth from dark energy between the Sun and the Earth is about $3.5N$.

8. May 2, 2015

### AgentSmith

It is not clear if gravity, or negative gravity, is the cause of the accelerated expansion or not. Brian Schmidt, one of the Noble laureates who used Type 1a supernovae to discover the acceleration contrary to his expectation, is not sure. We probably need new physics to tell.

9. May 2, 2015

### Chalnoth

It's gravity. Whether we're talking about the cosmological constant, or dark energy, or some type of modified gravity theory, the accelerated expansion is driven by gravity, as gravity is the only force that operates on very large scales.

10. May 2, 2015

### stedwards

Whether you may have misunderstood or not, could you provide a link to what ever it was Brian Schmidt was uncertain about?

11. May 3, 2015

### AgentSmith

I do not have a link handy, unfortunately. He and another astrophysicist discussed the reason(s) for the accelerated expansion in an online course on Cosmology. Of course, he discovered or co-discovered dark energy, the reason for the acceleration, but there is disagreement about its nature. Some call it quintessence, but that's playing word games IMO. Let me do some digging around for a link.

12. May 3, 2015

### AgentSmith

Gravity is not a force.

13. May 4, 2015

### Chalnoth

What is your basis of this assertion?

14. May 4, 2015

### Rob Benham

Bertrand Russell had a lot to say about the use of the term. We all know the sun doesn't set, but the expression is fixed forever.

15. May 4, 2015

### James Alton

This is a fascinating area of science for me that I hope to learn more about. As I understand things, the current expansion involves the creation of new space between distant objects such that the more space that currently exists, the faster the expansion. By extrapolating this simple fact, it becomes apparent that in the distant future, all of the galaxies that we currently see will eventually be moving away from us at greater than c and in effect disappear from our perspective. This would mean that space beyond our own galaxy could appear to be devoid of matter and black. What will the future astronomers (that lack the past history) derive from this view of the Universe? We seem to live in a particularly interesting period of time in ways. Thanks for all of the insight and please correct any misconceptions that I might have.

James Alton

16. May 4, 2015

### Chalnoth

In practice, the galaxies we can currently see will become more and more redshifted until they're nearly impossible to detect (this will take a *very* long time, possibly longer than life can exist, and certainly longer than the Earth will exist). We will never be able to see the galaxies evolve past the point that they left our horizon, however.

17. May 4, 2015

### AgentSmith

Gravity is a distortion in spacetime. Source: GR per Einstein(and many others).

18. May 4, 2015

### Chalnoth

It's still described as a force. General relativity doesn't change the usage of this term.

19. May 4, 2015

### Mordred

I'm not sure I agree with this . Cosmology and the EFE, FLRW metric are based upon GR and thermodynamic laws including energy density to pressure correlations. Pressure is force per unit volune

In terms of the EFE, and ideal gas laws the Cosmological constant may be modelled as opposite to gravity, but the EFE correlates the energy density and pressure correlations due to gravity by the stress energy tensor.

Treating the Cosmological constant as a form of force without specifying its pressure characteristics via

$$w=\frac{\rho}{p}$$.

The stress energy tensor to pressure and energy density is correlated via

$$T^{\mu\nu}=(\rho+p)U^{\mu}U^{\nu}+p \eta^{\mu\nu}$$ ( note in Minkowskii)

The equation of state for Lambda is a
W=-1. Cosmology should be treated in terms of the ideal gas law applications of the FLRW.

The equation of state for the cosmological constant is briefly covered here

http://en.m.wikipedia.org/wiki/Equation_of_state_(cosmology)

Last edited: May 5, 2015
20. May 5, 2015

### Chalnoth

Yes. But that pressure has no direct impact on expansion. However, pressure also acts as a source of the gravitational field, so that it impacts how gravity behaves.

This is probably made clearest by considering a box containing a negative-pressure substance, with zero pressure outside the box. The pressure would be pulling the sides of the box inward.

I think talking about the cosmological constant in terms of the pressure can be confusing. The simplest way to think of it is this: the curvature of the universe is a function of how much stuff there is in the universe. It manifests itself as the rate of expansion. Because the cosmological constant is constant, you get the same amount of stuff at all times, which means the curvature is constant, which means the rate of expansion is constant. A constant rate of expansion leads to exponential growth of the distances between objects.