Carbon Based

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exploringthecosmos:

If the universe is expanding, does that mean that I am getting bigger?
No. The electromagnetic forces that are holding you together are stronger than the expansion of the universe. This means that while the universe is expanding, you are not. Furthermore, while the space between galaxies is expanding, the galaxies themselves are not getting bigger; likewise with clusters of galaxies like the Local Group and even superclusters. The Virgo Cluster, for example, is ~100 million light years across and is marginally gravitationally bound. The attractive forces within the supercluster is slowed down by the expansion of the universe by approximately 20%.
Why is everyone leaving us? Does this mean that we are in the center of the universe?
It looks like everything is moving away from us because the universe is expanding! A useful tool to always keep in mind is the Cosmological Principle. This is commonly stated as ‘Viewed on a sufficiently large scale, the properties of the Universe are the same for all observers.’ This useful tool can be applied to the expanding universe. Since all observers in the universe (meaning your position in the universe will not affect one’s observations) will see the same expansion, everyone will see galaxies moving away from them! Following this, it seems like everyone would believe that they are at the center of the universe, but this can’t be. Every point in space is moving away from every other point in space; there is no unique center to the universe.
Is space expanding, or just galaxies moving apart in space?
Spacetime is constantly being created as the universe expands. With this expansion, galaxies seem to be moving away from us, but astronomers have a technique to test if galaxies are really moving. For the moment, imagine a really large sphere centered on our galaxy perhaps containing a few hundred galaxies. The galaxies along the surface of the sphere have two types of motion: random motion due to their own movement and motion due to the expansion of the universe. When each galaxy along the surface of the sphere is analyzed, they all have almost the same motion: the motion shared among each of these galaxies is due to the expansion of the universe and the motion that diverges from the motion which is caused by the expansion is random motion. It has been found that a significant proportion of the motion is due to the expansion of the universe and very little motion is random when we take into consideration galaxies at large distances. Thus, we can conclude that the motion of galaxies is indeed due to the expansion: the space between galaxies is moving, the galaxies are not receding in a classical sense. Going back to our tool, the Cosmological Principle, if the galaxies were indeed moving away from us, turning back time, the galaxies would be approaching a specific place in space and time where the Big Bang happened. This would violate the notion of a homogeneous and isotropic universe which would lead to the Big Bang happening in a particular place (ie: a special place in the Universe.) Since this cannot be the case, we can easily conclude that galaxies must not be receding away from us but rather that they appear to be.
Can recession velocity be greater than the speed of light?
Astronomers commonly probe the depths of space that reveal redshifts larger than one (z>1) which suggests that these observed objects are receding at greater speeds than that of light. Since nothing can move faster than light, we immediately know that these galaxies are not receding away from us with speeds greater than the speed of light. But if they seem to be moving away from us faster than the speed of light, what does this mean? The space itself is expanding faster than the speed of light! Recall that redshift is just the stretching of photons during their journey to our detectors. While the photons were on their journeys through space, space itself expanded faster than light which stretched these photons significantly to make the galaxies appear to be receding faster than light. Remember, nothing can move faster than light: except the expansion of space itself.
Where in space did the Big Bang happen?
Everywhere! And nowhere! To say that the Big Bang happened in a particular place in the universe would again violate the Cosmological Principle and the notion that there is no special place in the universe. Furthermore, the Big Bang created space, so to ask the question of where it happened is meaningless because prior to the creation of the universe, there was no space! The Big Bang was the creation of space and time whose spatial location in the universe has no meaning.
Are there any other questions you would like to see answered?

exploringthecosmos:

If the universe is expanding, does that mean that I am getting bigger?


No. The electromagnetic forces that are holding you together are stronger than the expansion of the universe. This means that while the universe is expanding, you are not. Furthermore, while the space between galaxies is expanding, the galaxies themselves are not getting bigger; likewise with clusters of galaxies like the Local Group and even superclusters. The Virgo Cluster, for example, is ~100 million light years across and is marginally gravitationally bound. The attractive forces within the supercluster is slowed down by the expansion of the universe by approximately 20%.

Why is everyone leaving us? Does this mean that we are in the center of the universe?

It looks like everything is moving away from us because the universe is expanding! A useful tool to always keep in mind is the Cosmological Principle. This is commonly stated as ‘Viewed on a sufficiently large scale, the properties of the Universe are the same for all observers.’ This useful tool can be applied to the expanding universe. Since all observers in the universe (meaning your position in the universe will not affect one’s observations) will see the same expansion, everyone will see galaxies moving away from them! Following this, it seems like everyone would believe that they are at the center of the universe, but this can’t be. Every point in space is moving away from every other point in space; there is no unique center to the universe.

Is space expanding, or just galaxies moving apart in space?

Spacetime is constantly being created as the universe expands. With this expansion, galaxies seem to be moving away from us, but astronomers have a technique to test if galaxies are really moving. For the moment, imagine a really large sphere centered on our galaxy perhaps containing a few hundred galaxies. The galaxies along the surface of the sphere have two types of motion: random motion due to their own movement and motion due to the expansion of the universe. When each galaxy along the surface of the sphere is analyzed, they all have almost the same motion: the motion shared among each of these galaxies is due to the expansion of the universe and the motion that diverges from the motion which is caused by the expansion is random motion. It has been found that a significant proportion of the motion is due to the expansion of the universe and very little motion is random when we take into consideration galaxies at large distances. Thus, we can conclude that the motion of galaxies is indeed due to the expansion: the space between galaxies is moving, the galaxies are not receding in a classical sense. Going back to our tool, the Cosmological Principle, if the galaxies were indeed moving away from us, turning back time, the galaxies would be approaching a specific place in space and time where the Big Bang happened. This would violate the notion of a homogeneous and isotropic universe which would lead to the Big Bang happening in a particular place (ie: a special place in the Universe.) Since this cannot be the case, we can easily conclude that galaxies must not be receding away from us but rather that they appear to be.

Can recession velocity be greater than the speed of light?

Astronomers commonly probe the depths of space that reveal redshifts larger than one (z>1) which suggests that these observed objects are receding at greater speeds than that of light. Since nothing can move faster than light, we immediately know that these galaxies are not receding away from us with speeds greater than the speed of light. But if they seem to be moving away from us faster than the speed of light, what does this mean? The space itself is expanding faster than the speed of light! Recall that redshift is just the stretching of photons during their journey to our detectors. While the photons were on their journeys through space, space itself expanded faster than light which stretched these photons significantly to make the galaxies appear to be receding faster than light. Remember, nothing can move faster than light: except the expansion of space itself.

Where in space did the Big Bang happen?

Everywhere! And nowhere! To say that the Big Bang happened in a particular place in the universe would again violate the Cosmological Principle and the notion that there is no special place in the universe. Furthermore, the Big Bang created space, so to ask the question of where it happened is meaningless because prior to the creation of the universe, there was no space! The Big Bang was the creation of space and time whose spatial location in the universe has no meaning.

Are there any other questions you would like to see answered?

Filed under universe astronomy cosmology Big Bang science

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