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Is the age of the universe in the CMB frame the maximum it can be in any reference frame? If not, in what sort of reference frame could it be older?

In a similar (and possibly dumb) question: When we say that the expansion of the universe is accelerating, is it possible that the objects at the far edges of the universe are actually travelling at constant velocity from their reference, but as they get further away from the gravitational center of the universe, the time dilation reduces what a second is and therefore appears from our reference to be accelerating?

E.g. if a=dv/dt. If we measure the velocity of the distant object for an arbitrary amount of time - say 10,000 seconds. And let’s say the object is travelling at constant velocity, but as the object gets away from the gravitational center, a delta 10,000 seconds from the object’s reference would be shorter than a delta 10,000 seconds from our reference. So therefore we perceive this as a change in velocity?

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>But what keeps the train moving? I know the answer to this question is inertia, but intuitively it makes sense that there must be some force that is making the object continue to move, even at a constant velocity. I guess a better question is do we know why objects with no net force can remain in motion? Like, it makes sense to me that when net force = 0 = no net movement, but not the constant velocity part.

Why is it that when you're standing inside a train (or airplane or car) moving at constant speed, you move along with the train without having to constantly horizontally push on the floor?

According to your reasoning, you're moving at constant velocity and that means you need to be pushing on something to keep moving forward. But actually you don't have to push on anything. Does that tell you anything about your intuition with respect to motion in different frames of reference?

You may also want to contrast this experience with your experience on something like a merry-go-round, where you know that unless you are actively exerting force against a pole or something else on the merry-go-round, you'll fall off. Do you know what the key difference between these situations is?

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Perhaps I should say there's no unique choice for a universal "now". "Now" would be a 3D surface in 4D spacetime that intersects the point "earth in the year 2023". But there are lots of possible choices you could make in defining this surface, and none of these choices are more correct than the others.

Others have a lot more competence to answer than me, but I'll mention the existence of an elegant hypothesis, https://en.wikipedia.org/wiki/Eternal_inflation , the eternal inflation.

A TL;DR would be the "what if" the cosmic inflation, after the big bang, wasn't properly identical in every direction and, well, while areas "cooled off" most others remained subjected to inflation for a while longer, etc, leading to a weird multiverse or perhaps oddly patchwork-based idea of a universe or multiverse. Our universe being one of the areas in which inflation ended up before the rest, even though, most likely, the largest part of the matter and energy from the big bang is still in its inflation phase; we'd be like shards off, with a headstart.

And in such a context, the answer to your question would be a weird "no".

It may be scientifically-based, but it's perhaps less provable than a science-fiction novel, so it's to be taken with a grain of salt, but, eh, it's still an original take on the answer to give you, hehe, I always found it fascinating that such notions will be possible to imagine. It doesn't need to be true to be awesome.

I’m sorry could you explain that more please? There is no universal “now”?

How do hunters distinguish them?

I recall in my sedimentology course one exam where we were given buckets of sand. We had to identify the approximate location of the sources of each sample within a few miles. I don't remember the specific distance, but it was surprisingly small and the sand samples came from basically anywhere in the U.S.

At the beginning of the course we all understood that by the end we would be asked to do this and it was intimidating. But by the end of the course and in practice, we were all pretty able to make the location identifications accurately. This was just an undergraduate course in sedimentology and composition of the sand grains where easy enough to separate identify the composition of and at what percentages. If I recall correctly.

Other types of rocks, igneous and metamorphic would use different methodologies for identification. Now you picked that rock up from a beach in Ireland. It's just one rock, how did it get there by what action? Waves, maybe a glacier or two, rivers? Because Ireland is an island I'd greatly suspect that it's from Ireland but with glaciation it's conceivable that it's path to that beach started out at a granite or metamorphic rock formation further north from where you found it. Heck when you involve the actions of glaciers, your rock could have started out in Scotland, but probably not. It could be a simple black basalt from the ocean bottom. That would be difficult to identify, but even those have different properties that a lab with the right comparative samples could identify.

Since you found your keep sake on a beach it still could have formed anywhere and was transported there by many actions over millions if not billions of years. It could have been a gastrolith in the belly of a dinosaur that migrated across the continent when there were land bridges. I just googled to see if there ever any dinosaur remains found in Ireland and was somewhat surprised to find that there are a few. But the odds of your rock being a gastrolith from the belly of an Irish dinosaur is about the same as both you and I winning the jack pot in the lottery.

Rocks are awesome things, and it's possible to differentiate your rock from any that might be found here. But when we are just dust millions of years from now, someone/something could pickup your rock again and ask a similar question about it. So now it's an American rock that currently holds your memory of Ireland. I love Ireland.

The CMB frame is different in different places. It's also the frame of a comoving observer -- that is, one who is moving only due to the expansion of the universe and does not have further ("peculiar") motion. So if we consider distant galaxies in some direction, they are receding in that direction, and so is their CMB frame.

Yes, that is just inertia by definition (see below definition from dictionary.com) "the property of matter by which it retains its state of rest or its velocity along a straight line so long as it is not acted upon by an external force."

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Does the cosmic background radiation have a different velocity relative to the earth in different directions, or as far as we can tell we can consider all of the CMB in every direction to be identically “stationary”.

You're confused because you think "at rest" and "in motion" are different things, but they're not. They're the same.

Imagine two space ships, Firefly and Enterprise, in deep space set on a collision course. The captain of the Enterprise radios the captain of the Firefly saying, "Firefly, you're moving towards us at 1000m/s. Please engage reverse thrusters to match our velocity or alter your course." The captain of the Firefly responds, "No, Enterprise. YOU'RE moving towards US at 1000m/s. You engage YOUR reverse thrusters or alter YOUR course."

Who is right? Both, and neither. Both are correct from their own inertial reference frame, and both are incorrect from the other's inertial reference frame.

It's not entirely certain. There have been no human cases of CWD, but it is known that CWD poses a threat, most likely, to non-human primates. CDC recommends not eating meat from deer infected with CWD, as it is possible that CWD could potentially spread to people, similar to Mad Cow Disease.

I read almost all your replies Kwooosh, the keyword you are using is "intuitively". The problem is that since we lived our whole life on earth where friction happens all the time this make sense for this topic to feel counter-intuitive. Newton law is a mathematical model and it could be counter-intuitive sometimes.

It's the center-of-momentum frame for all of the CMB photons (within some volume of space), not the frame of individual photons. It's also the frame in which the CMB temperature is the same in all directions. If you're moving with respect to that frame, you'll find that the CMB is hotter (blueshifted) in the direction of your motion and colder (redshifted) in the opposite direction. That's what we find, and the magnitude of this effect tells us that we are moving at 370 km/s with respect to the CMB.

Not only is that information old, but you will never be able to receive information from those places as they are today. They are moving from us faster than the speed of light due to the expansion of space, and will never again be connected to us (unless of course the expansion is only local in the larger universe and will later reverse, or some other unknowns). It's groovy to think about.

How is the rest frame of the CMBR determined if light travels at the same speed in all reference frames? Is it to do with the red and blue shift?