# Am I expanding?

1. Apr 25, 2005

### icvotria

I'm possibly being a little naive here, but if the universe and everything in it is expanding, does that mean that all the particles in my body are expanding (if perhaps at a negligible rate)? Quite petrifying....

2. Apr 25, 2005

### SpaceTiger

Staff Emeritus
No, you're not expanding, but there should be a very tiny force on the components of your body due to the expansion of space. This force will be swamped (by many, many orders of magnitude) by the electrostatic forces of the molecules holding your body together.

3. Apr 25, 2005

### KingNothing

So in short, yes.

4. Apr 25, 2005

### SpaceTiger

Staff Emeritus
The tiny force would not result in a net expansion of your body, it would simply be included in the equations that govern the atoms that hold your body together, making you very slightly less tightly bound. As an analogy, imagine you're pulling lightly up on a heavy object. The net force will still be down, so the object won't move, but its total weight will be decreased a bit.

5. Apr 25, 2005

### icvotria

So I'm not expanding, this is good. But the fact that the universe is trying to expand me is possibly a bit more creepy. Thanks for your replies.

6. Apr 25, 2005

### pervect

Staff Emeritus
The tidal forces of the Earth are also stretching you in one direction (your height, specifically, at least when you are upright - because your head is further from the center of the Earth than your feet are) - though it turns out the Earth's tidal forces compress you in the other two directions.

This is a _much_ bigger effect than anything due to the expanding universe. The numbers for the cosmological tidal force were actually calculated in another thread,

https://www.physicsforums.com/showt...&pp=15&highlight=hubble+constant+acceleration

the answer turns out to be (assuming standard isotropic cosmological models)

acceleration / unit length = -q H^2, where q is an odball cosmological parameter called the "deceleration parameter", and H is the Hubble constant.

the numerical value of H is in some debate, and q is even harder to measure, but with the assumptions

With q = -0.6 and H = 71 Km / s Mpc, the cosmological tidal force is

3.12 Ã— 10-33 m / s-2 for every km.

(calculations courtesy of hellfire)

for comparison, the tidal stretching forces for an object on the Earth's surface are 1.53 e-3 m/s^2 per kilometer, making them 30 magnitudes of order stronger.

7. Apr 25, 2005

### icvotria

Are there any more forces trying to distend my shapely form that I should know about?! I'm going to curl up in a lead box and weep. Only joking. It's funny that you should say that the value of H is widely debated cos I just came across it in class the other day and was told it's 65kms-1 Mpc. I'm going to stick with my value cos it means I'm being expanded less.

8. Apr 25, 2005

### SpaceTiger

Staff Emeritus
Air pressure fluctuations and differentials, radiation pressure, thermal expansion and contraction, fluid pressure from blood flow, external magnetic fields, and many more. All of these forces will be much stronger than the expansion of the universe.

WMAP gives 71 +-4. Most people believe that, but there are still a few dissenting voices.

9. Apr 25, 2005

### KingNothing

It depends on how you define the expansion. Are we assuming that the universe is expanding on the non-atomic level (i.e. atoms are moving farther apart to create more volume) or are we assuming that everything is expanding (i.e. even atoms, creating a bigger volume). With the latter definition, atoms and molecules wouldn't need to come apart (I agree with you that they wouldn't).

10. Apr 25, 2005

### SpaceTiger

Staff Emeritus
Well, in theory, everything should be feeling the force of the expansion (including the interiors of atoms), but one might need to invoke a theory of quantum gravity to explain the extremely tiny force that an atom would feel from it. I'm not even sure how to properly approach that problem. It's best to just rest assured that the effect is so negligible that it's not even worth considering on human scales.

11. Apr 25, 2005

### icvotria

Am I right in thinking that the force of expansion is increasing? Does this mean that eventually everything will be entirely "expanded"? Could it get strong enough to split everything up into it's tiniest form? Sorry I don't know any of the correct terms for anything, I'm trying to get by on a third of an a-level and very little background reading.

12. Apr 25, 2005

### SpaceTiger

Staff Emeritus
Current theories don't predict that the forces from the expansion will ever overcome the forces holding your body together. Only in a universe in which the dark energy has a negative equation of state will a "big rip" (the term for what you're describing) occur.

13. Apr 25, 2005

### Spin_Network

From Birth you have expANDED..and if you grow old you will be COMPacted...
the volume of your total mass is forever changing (because you interact with other atoms..like taking in energy as food fror instance..this exerts change in body)..its dynamical..so is the Universe!

14. Apr 25, 2005

### KingNothing

Your response had absolutely nothing to do with the question. The thread starter was not talking about aging, or changes in volume because of physical change in the amount of matter. He was asking a question about the change in volume without the addition or subtraction of new matter.

EDIT: And you are wrong, to boot.

15. Apr 26, 2005

### Spin_Network

Volume expansion without additional matter is INFLATION!

Expansion of atomic particles is a badly worded question, if you expand molocules, then you have a net gain of Atomic Particles.. [bonds]..you have expanded the area and Volume of a collection of Molocules. Likewise a net loss of Atomic Particles will mean one is contracting to a finite area.

So really is a question of Inflation?

16. Apr 26, 2005

### aviator

if everithing was expanding at the same rate you wouldnt notice anything expanding because you would be expanding as well as other things but you would notice a push (gravity)

this would create a force of atraction between objects, an expanding earth would push objects while expanding towards the inside

then it would be volume not mass what would determine the force of atraction

but this doesnt explain the orbites of planets that trully would be spiral, i tend to consider this spiral orbitation as time

17. Apr 26, 2005

### Danger

Just be really careful with the contraceptives and you'll be fine. You won't explode or anything.

18. Apr 26, 2005

### marlon

:rofl: Thus spoke Dr Danger, Professor in Bio-AstroPhysics

marlon, phd student of Dr Danger

19. Apr 26, 2005

### Danger

I hate getting so technical, but sometimes there's just no other way... :tongue:

20. Apr 27, 2005

### KingNothing

Okay, we can call it that if you want.

Not necessarily, unless you assume that there is no 'empty space' within an atom.