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CassiopeiaA
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A very fundamental doubt. Was inflation expansion of just space or the universe with whatever the state of matter it had?
The expansion of space is indistinguishable from the increase in distance between objects within space. They're two ways of describing the same phenomenon.CassiopeiaA said:A very fundamental doubt. Was inflation expansion of just space or the universe with whatever the state of matter it had?
CassiopeiaA said:A very fundamental doubt. Was inflation expansion of just space or the universe with whatever the state of matter it had?
Inflation can only occur if the universe is dominated by the right kind of energy density. This energy density needed to be uniform across the initially inflating region of the universe, and it needed to have the peculiar property that the density remained constant as the universe expanded. This constant energy density results in the hallmark accelerated expansion of inflation.CassiopeiaA said:A very fundamental doubt. Was inflation expansion of just space or the universe with whatever the state of matter it had?
bapowell said:This energy density needed to be uniform across the initially inflating region of the universe, and it needed to have the peculiar property that the density remained constant as the universe expanded.
It's not quite constant, but it does change very slowly. The super-short explanation is that there's a quantum field (called the inflaton) that has some potential energy. At some point, it takes on a particular value, and that value is represented by a specific energy density due to that potential energy. The tendency of the field is for its value to change in the direction of lower energy density. However, the expansion actually acts as a sort of friction, preventing the field value from changing quickly. If the parameters are balanced in the right way, such that this friction term is large, then this results in a nearly-constant energy density, which causes inflation.CassiopeiaA said:The energy density should be uniform, but how can it be constant during inflation?
Energy is not conserved in general relativity.CassiopeiaA said:The energy density should be uniform, but how can it be constant during inflation?
Chalnoth said:The expansion of space is indistinguishable from the increase in distance between objects within space. They're two ways of describing the same phenomenon.
Right. Which is equivalent to saying the distance does not increase.aboro said:Thus, there is no expansion occurring between the moon and the earth.
mfb said:Right. Which is equivalent to saying the distance does not increase.
Expansion of space and increasing distance are equivalent, that is the point. If one is zero then both are.
aboro said:With all of our technology, we know that the the distance between the Earth and the moon is 298,900 miles. If that distance is increasing because the Universe is expanding, is it taking light a longer amount of time to travel from the Earth to the moon or has the light red-shifted as compared to the light that was traveling between these two objects a billion years ago?
Drakkith said:Expansion is not causing the Earth and the Moon to move away from each other. Gravity is strong enough to prevent it. .
The cause of the current expansion is the initial conditions for our universe. Those initial conditions are as yet unknown. The matter and energy in the universe (including dark energy) influence how this expansion changes over time.aboro said:My understanding about the expansion of the Universe is that it is observed when assessed at great distances (as between two galaxies) and that the cause of the expansion in unknown although the consensus is that dark energy may be the force that is causing galaxies to repel from each other. How do we know that the (unknown) forces at play in causing distant galaxies to be moving away from each other (i.e., being repelled from each other) is the same force that is allowing the moon to remain in sufficient equilibrium so as to only be affected by tidal forces and the Earth's angular momentum?
Chalnoth said:Bound systems like our galaxy don't have to compete with this expansion because the inertia of the expansion was overcome long ago when the system was first formed.
Sort-of-inertia. Not the inertia of objects in space, but the evolution of spacetime itself.aboro said:If I understand you correctly, you are saying that dark energy is not the cause of the expansion of the Universe. Inertia is.
That does not make sense.aboro said:If our galaxy no longer has to compete with the force of the Universe's inertia because that force was overcome long ago
Inertia isn't a force. It's just a statement of Newton's first law, "An object in motion tends to stay in motion." The universe is expanding today because it was expanding yesterday. No force is required for the expansion to continue.aboro said:If I understand you correctly, you are saying that dark energy is not the cause of the expansion of the Universe. Inertia is.
If our galaxy no longer has to compete with the force of the Universe's inertia because that force was overcome long ago, is it still correct to say that this force does not and cannot cause the distance between the Earth and the moon to expand? Stated differently, the only force affecting the distance between the Earth and the moon is gravity. For the same reasons why the existence of the "ether" was repudiated by Einstein, cannot the same be said with respect to the presence of the Universe's inertia within our own galaxy (and other bound systems as well)?
CassiopeiaA said:The energy density should be uniform, but how can it be constant during inflation?
Chalnoth said:It's not quite constant, but it does change very slowly. The super-short explanation is that there's a quantum field (called the inflaton) that has some potential energy. At some point, it takes on a particular value, and that value is represented by a specific energy density due to that potential energy. The tendency of the field is for its value to change in the direction of lower energy density. However, the expansion actually acts as a sort of friction, preventing the field value from changing quickly. If the parameters are balanced in the right way, such that this friction term is large, then this results in a nearly-constant energy density, which causes inflation.
If you want to read up on it in more detail, look around for, "slow roll inflation."
bapowell said:Energy is not conserved in general relativity.
In the eternal inflation picture, while the field tends towards a lower energy state, quantum fluctuations bounce it around a little bit on its way there. So some regions increase a little bit in energy while others decrease. Any given region is more likely to decrease in energy than increase, but those that increase in energy expand faster and take up more volume in the end. If the parts that increase in energy expand quickly enough, then this process can continue forever.slatts said:I think if I can put Chalnoth's and Bapowell's replies to CassiopeiaA's last question together, I'll be able to resolve some confusion I've had about descriptions of inflation models that are eternal to the future. Those descriptions have been plain English, but it's sounding like some exponent in the equations they've been talking about must be asymptotically approaching "1" (approaching "1" without ever quite reaching it). Could anyone familiar with mathematical descriptions of some of the eternal inflation models tell me whether I might be correct, or, if not, how any decrease in the density of the energy involved could otherwise continue eternally without fading into the purely inertial expansion that those multiverse models describe as being confined to their "local" (or "pocket", or "bubble") universes? (If the explanation involves derivatives rather than exponents, I should admit that my knowledge of them is still at the Wikipedia level.) Thanks.
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Inflation is a theory in cosmology that explains the rapid expansion of space in the early universe. It suggests that the universe underwent an exponential expansion in the first fraction of a second after the Big Bang, causing it to grow from a tiny, dense point to a much larger size.
There are several lines of evidence that support the theory of inflation, including the cosmic microwave background radiation, which shows a uniform distribution of temperature across the universe, and the large-scale structure of the universe, which appears to be remarkably uniform. Additionally, observations of the cosmic background radiation have revealed slight temperature variations that align with predictions of inflation.
The horizon problem is the question of why the temperature of the cosmic microwave background radiation is so uniform across the observable universe when there has not been enough time for light to travel and equalize the temperature. Inflation solves this problem by positing that the rapid expansion of space during inflation allowed for different regions of the universe to come into contact and reach thermal equilibrium before the expansion of space separated them again.
The Big Bang theory describes the universe as starting from a hot, dense state and expanding over time. In contrast, inflation is a specific period of rapid expansion that is thought to have occurred in the first fraction of a second after the Big Bang. Inflation helps to explain certain aspects of the universe that are not accounted for by the Big Bang theory alone, such as the uniformity of the cosmic microwave background radiation and the structure of the universe on a large scale.
While inflation is currently the leading theory to explain the expansion of space, it is still a subject of ongoing research and debate among scientists. While there is strong evidence to support the theory, there are also some inconsistencies and unanswered questions. As our understanding of the universe continues to evolve, it is possible that new evidence or theories may emerge that challenge or refine the concept of inflation.