# New to the board. Gravity questions

• tommac
In summary, Tom has a question about the relationship between gravity and the expansion of the universe. He wonders if empty space can be curved and if there can be a gravitational field that permeates the entire universe. He also considers the possibility of space having a natural 4-dimensional curve. However, it is explained that the presence of energy is needed to curve spacetime, and in truly empty space there is no energy. Tom also asks how much gravity would be needed to create the redshift effect and how to calculate and test this. It is noted that the dynamics of the universe are determined by the density of material, even if it is dispersed evenly. Finally, the concept of zero point energy in empty space is discussed and it is concluded that

#### tommac

Hi,

My name is Tom. I am just learning about space and physics and how the universe works.
I have a question and I am not sure if I am posting it in the correct place or not so please let me know if I am posting it wrongly.

My question is about gravities relationship with the expansion of the universe.

The question is that could empty space be curved?

By this I mean either:
1) Can there be a gravitational field that permiates the universe as a whole. Maybe from the sum of small ( negligable effects ) multiplied over the numerous massive of the universe, dark matter, or using brane theory ... a source from outside our brane.
2) That space just has a natural 4d curve to it.

Now for number one above. If all of the matter in the universe has an effect on every single point in the universe ( even if it is negligable ) then at each point all forces would roughly even out so the direction of the gravitational pull would not exist, however the effect of the red shift would work.

Now the questions remains how much gravity would be needed to create the redshift needed for this effect. Since the curvature near a massive object greatly offsets the effects of universal expansion, I think the amount of curvature throughout space needed to produce the redshift should be within reason.

My question is how can I calculate the amount of gravity that would be needed?
How can I test this?

tommac said:
Hi,

My name is Tom. I am just learning about space and physics and how the universe works.
I have a question and I am not sure if I am posting it in the correct place or not so please let me know if I am posting it wrongly.

My question is about gravities relationship with the expansion of the universe.

The question is that could empty space be curved?

This is a good question. The answer is a resounding no. Empty space cannot be curved. In the theory of General Relativity the presence of energy causes curvature in spacetime that leads to the effects that we call gravity. In empty space there is no energy hence no curvature.

tommac said:
By this I mean either:
1) Can there be a gravitational field that permiates the universe as a whole. Maybe from the sum of small ( negligable effects ) multiplied over the numerous massive of the universe, dark matter, or using brane theory ... a source from outside our brane.
2) That space just has a natural 4d curve to it.

To continue the answer from about, spacetime doesn't have a 'natural' curve to it, as I say it takes the presence of energy (of which matter is one example) to curve spacetime.

tommac said:
Now for number one above. If all of the matter in the universe has an effect on every single point in the universe ( even if it is negligable ) then at each point all forces would roughly even out so the direction of the gravitational pull would not exist, however the effect of the red shift would work.

Now the questions remains how much gravity would be needed to create the redshift needed for this effect. Since the curvature near a massive object greatly offsets the effects of universal expansion, I think the amount of curvature throughout space needed to produce the redshift should be within reason.

My question is how can I calculate the amount of gravity that would be needed?
How can I test this?

Note that redshift is produced by a combination of the receding motion of distance galaxies (the doppler shift) and the gravitational effect of the energy in the Universe acting on photons as they travel.

Now there is something important to realize. It might seem like in an infinite universe with matter spread out evenly then the effect of gravity of everything will cancel out and gravity plays no role. It turns out that this is not the case, not because of some strange effect of relativity, even in Newtonian gravity the same things happens. So the dynamics of the Universe is determined by the density of material in the Universe, even if it is dispersed evenly.

Wallace raises a point worth considering. In an observationally finite universe, CMB uneveness [anistropy] is guaranteed. In an observationally infinite universe, perfect gaussian distribution is guaranteed.

Wallace said:
In empty space there is no energy hence no curvature.

Wouldn't ZPE effect "empty" space and give it a non-zero amount of energy?

Yes. But truly empty space has zero zero point energy, i.e. the cosmological constant is zero. If the CC is not zero, then the space is not 'empty' from the point of view of cosmology.

## 1. What is gravity?

Gravity is a natural force that causes objects with mass to be attracted to each other. This force is responsible for keeping planets in orbit around stars and objects on Earth from floating away into space.

## 2. How does gravity work?

According to Einstein's theory of general relativity, gravity is caused by the curvature of space-time around massive objects. The more massive an object is, the more it curves space-time, resulting in a stronger gravitational pull.

## 3. Why do we feel gravity on Earth?

We feel gravity on Earth because the planet has a large mass, which curves the space-time around it. This causes objects to be pulled towards the center of the Earth, giving us the sensation of weight.

## 4. Can gravity be turned off?

No, gravity is a fundamental force of the universe and cannot be turned off. However, its effects can be reduced in certain situations, such as in space where there is less mass and therefore less gravitational pull.

## 5. How does gravity affect time?

According to Einstein's theory of relativity, gravity can affect the flow of time. The closer an object is to a massive body, the slower time will pass for that object. This phenomenon is known as gravitational time dilation.