121–130 of 380 posts
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Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?
You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.
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> The assumption made here is that relative velocity is the only method that would redshift light. Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime. > Gravitational redshift is a thing Not for the universe as a whole, no. Gravitational redshift is only meaning…
>Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating. Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?
The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, since both types of stress-energy are allowed for in the theory.
"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. "Dark energy" only poses a problem if for some reason you don't like having a nonzero cosmological constant.
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We know for sure that there are issues with our current theories. Our two best theories, general relativity and quantum mechanics, are not compatible with each other. https://en.wikipedia.org/wiki/General_relativity#Relationshi...
Interesting to think we only got the “middle” size physics right, but we can’t reconcile it with micro or universal physics.
planetary have very important applications!
But I'd argue not nearly as many as "Here are all the formula that govern a falling apple."
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Well for one, if the laws of physics are not time invariant (i.e. the laws of physics are not the same at all points in time) then energy can be created or destroyed [1]. So that would be quite a shocker. [1] https://en.m.wikipedia.org/wiki/Noether%27s_theorem
Doesn't dark energy do exactly that? 70% of the energy of the universe doesn't seem to want to play by our rules!
Can someone help me understand - point me toward some reading or ELI5 - what is the universe expanding into? (Or probably, why is that question not formulated well?)
I've asked this question before, but I don't think I received a good answer, so figured I'd try asking again. How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration. However, wouldn't we expect to see the same observation even without any…
-Galaxies are not accelerating, space is expanding.
> Imagine there are some objects in space all moving in random directions and speeds, relative to Earth...
-No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood.
I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is).
They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity.
There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.
It is very fortunate that the universe is expanding. This provides a virtually unlimited source of energy.
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Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?
You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.
With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”;
diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection;
and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric;
and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion,
with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?
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>Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating. Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?
I'm not sure what you mean by "phantoms in the data". The distribution of stress-energy is a free parameter in GR; it has to be inferred from observations. The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, sin…
It's clear to me why we haven't made any new discoveries in cosmology in the past two decades. It's this exact attitude of "the model is the truth". All models are wrong. The data can help you improve it, but you have to at least want to improve it.