Stars in distant galaxies are typically more massive
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Edit: and this from last year https://nbi.ku.dk/english/news/news21/danish-student-solves-...
》In other words, climate change makes the differences in temperature grow locally — and with large temperature differences come even more extreme weather patterns.
Makes a lot of sense.
I don't think that, in this regard, you can say much more than "the universe is not isotropic across space-time".
The cosmic microwave background gives you a physical realization of just that (but of course only approximately so), at least as far as cosmologists are concerned. The rules of relativity of course still apply, making this particular synchronicity convention just one of many others...
An event is a single point in spacetime, whereas photon decoupling happens everywhere, defining a spacelike hypersurface we use for synchronization (in the idealized scenario).
Subsequently, the CMB allows us to single out a particular reference frame (the one where it looks isotropic) and provides a measure of expansion via its redshift/temperature which we can then translate to cosmological time (ie time since the big bang as measured by an observer following the Hubble flow) via our cosmological models.
What Relativity forbids is anything traveling as fast as c, as the increase in mass as c is approached would require infinite energy to reach. Relativity says absolutely nothing about faster than c, i.e. it does not forbid things such as the theoretical tachyon.
So, more distant means we are viewing events that happened further in the past.
We are (almost certainly) not at the center of the universe, but we are, by definition, at the center of our observable universe.
In this respect, you are the centre of your universe. But objectively speaking, there is no centre.
https://en.wikipedia.org/wiki/Vector_clock
Take a photon starting from your chest and give it t1.
* Assume that the speed of light is symmetrical. *
When the photon hits the next person's eyes, it is t1 + distance/speed of light.
Meaning, the photon is "from the past".
Since all photons arriving are from the past, the photons travelling the shortest distance are the newest (representing the largest timestamp).
So, the thing with the largest observable timestamp is yourself.
The confusion arose because the OP said "you are the oldest thing in the universe you can observe". They should have said "you are the most recent thing in the universe you can observe".
If you're hearing thunder now, in a way, you're hearing an event in the past. The farther the thunder traveled, the farther into the past you are experiencing.
Across the expanse of space, you are not just hearing, but seeing into the past. The farther the light traveled, the farther into the past you are experiencing now.
By this logic, I'm the newest thing in the universe, because it takes the shortest time for me to observe myself.
The fact is, simultaneous observations don't necessarily mean simultaneous events. If you hear 2 people shouting at the same time, but one person is very far away and one person is near, you can assume the person farther away actually shouted first. The reason this happens is the different delay across different distances.
The distances have to be much greater to notice the difference with light compared to sound. But compared to the expanse of the universe, light actually travels quite slowly. If the sun disappeared, we wouldn't see it disappear on Earth for 8 minutes (that's how long it takes light to reach Earth). Essentially, we are seeing the Sun as it was 8 minutes in the past.
The effect is even greater outside our solar system and even greater beyond our galaxy. We see stars and galaxies as they were, not just minutes but years and years in the past. Some stars that we see now have certainly already reached the end of their life, exploded into a supernova and collapsed. We just haven't seen it yet. The light from those events haven't reached us yet.
Everything we see is of something as it was in the past. Even though we are seeing all the stars and galaxies simultaneously, the farther away it is, the farther into the past we are observing.
Not according to the classical definition of age which is the time since birth. I was born a few decade ago. My grandparents are decades older then me. The earth in millions of year older than me. And the galaxies billions of lightyears away are still billions of year older than me.
Maybe the Milkyway is older than the other galaxies we observe, assuming they were all born at the same time.
We're not talking about the classical definition of age.
The moon is older than we are, due to gravitational time dilation. So is Mars. Even if you account for the time it takes for light to travel to here from there.
My remark was intented as an explanation of one of the parents' statement: "distant means younger" when observing astronomical objects. We are not talking here about the age of the object per se (how old is the star I see, etc.), but about the age of the observation (how long ago did that happen, what I see just now). Every look into the sky is a look into the past. But also every look around you is a look into the past.
With "the oldest thing in the universe", I did mean the particles that form you (not you as a specific formation of such particles). If we say that everything of the universe started at the same time, then the atoms closest to you are the oldest things in the universe you can observe. Objectively, of course, every particle has the same age, even though it might be beyond the event horizon and never be observable. (I am simplifying somewhat here, leaving the spontanous coming and going of particles in the vaccum out of the picture.)
All places are equally old.
So a galaxy a billion light years away is as old as ours. But we see it as it was 1 billion years ago.
How do we know where the Big Bang occurred?
Unles it means that our galaxy is on the spherical plane(?) directly projected out in all directions from the bang, and it’s the diameter of this sphere that is billions across?
This is true for anything we don’t know.
For the Big Bang, we know the intermediate problem: black holes. Gravity and quantum mechanics interacting at the same scale. The singularities there are accessible in a way the singularity of the Big Bang is not.
We can never know the answer, because any possible cause was outside of our observable universe. Inside of our universe, time started at the big bang, so there was no 'before' and thus there was no cause, because the cause needs the time to be before the effect.
[0] https://www.independent.co.uk/space/universe-expanding-colla...
I mean, obviously this is nonsense, but... I can't exactly disprove it.
To mention the obvious: everything is constantly moving through the time dimension together, for unknown reasons. Nothing like that happens with space.
Wish we could figure out some better ways to experiment with time. Seems like it's very telling of what's really going on that it's so different.
Our intuitions about physical reality come from evolving as primates on Earth. We're pretty great at intuitively understanding the world at that scale.
But at vastly different scales of size and time, those intuitions just don't apply much. So quantum physics events on nano -meter and -second scale seems completely bizarre, as do cosmological event on giga -lightyear and -year scale.
So, extrapolating this in the past, at some point the universe must have been really tiny, and then grew much larger. But this growth happened essentially everywhere: spacetime itself expanded and pushed things apart at speeds greater than the speed of light (which is allowed if the coordinate system itself is what's changing), leading to objects which were once microns apart now being tens of billions of lightyears away in just a few billion years.
Isn't it our observable universe which must have been really tiny? the universe itself could have been infinite in its early stage as well right?
Common knowledge is that there was an initial event called the Big Bang but there's no observational or theoretic evidence that there was such an initial event or explosion. Observations are limited by cosmological horizon, and our theories are unable to describe the state of the universe in its "early on" stage.
What we do know however is that the universe is expanding and getting less dense and colder as a result. By going backward, we interpolate the state of the universe far in the past up to a stage where it was very dense and very hot "soon after" an hypothetical instant zero that we don't know happened.
Also at that stage, the universe may have been infinite already (provided it's infinite now). Maybe it's better to think of the Big Bang has a past state of the universe rather than an "explosion" that happened at given time and place.
Why? Let's reduce by one dimension to be able to imagine this: imagine a small thing (a grain of sand) explodes in our 3D world and all the tiny pieces are flying away at equal speed, then the structure that is created by the explosion is not a 3D sphere, but the surface of a sphere, a 2D structure, because where the explosion started, there is nothing left of the original thing, as every particle of it has moved away from that point, so at any point in time after the start of the explosion, the particles are on the surface of a sphere. That surface is a 2D structure embedded into our 3D world, much like the surface of an expanding balloon. In the same way, our universe is a 3D structure, so looked at from the outside (which we obviously cannot do), it is not a 3D sphere, but probably more like the surface of a 4D sphere.
Now imagine us as a dot on a balloon that is expanding, and other stars are also dots on the surface of the balloon. In whichever direction you look, all the stars are moving away from you (the balloon is expanding). You do not need a center of this structure to observe this moving away -- nothing is in the center -- there is no center.
So from our point of view, we can never know if universe is actually infinitely distributed or not.
Then again, there's the matter of looking so far back that you can't see stars because it's so old and thus it's not that there is an edge but just that we can't observe one.
That would require stars to be outputting infinite energy in the form of light we can see. They are not, so we do get enough of their light to see them with the naked eye. The farther away they are, the less and less photons make it to our eyes. So, yes, you can look up in the night sky and see infinite galaxies, but your eyes are not sensitive enough to pick out the one or two photons every few seconds and tell you there is something there.
However, we have tools to do that for us: https://www.space.com/17755-farthest-universe-view-hubble-sp...
if the 4th dimension is time, the argument implies that the center is "a long time ago"
You and I both see that the surface area of a sand grain, distributed by explosion to a different surface area with the diameter of a beach ball, has a center in 3D space.
But from the perspective of a Flatlander on the expanding surface, looking at the distance to an adjacent microbe, the universe is growing, and there is no meaningful point on that 2D surface from which the expansion emanates, it's expanding everywhere.
There's no reason to believe our universe is not infinite. It's certainly effectively so.
This means that the tip of your nose, a distant mountaintop, Olympus Mons on Mars and indeed a distant galaxy were all the same space.
Imagine a balloon blowing up with an ant on it. The ant can only walk so fast but the balloon can blow up expanding the space between ants in a way that is in no way limited by ant walking speed.
Now that we are so separated it takes so long for light to get to us that we can only see the light that left billions of years ago from our frame of reference
I don't think it is entirely ruled out, but two causes of redshift are considered less likely than one.
Is that right? I thought the really, really big stars in the early universe burned-out in as little as a few million years.
I know one reason: metallicity.
The very early universe didn’t have clumps of heavy elements to create nucleation points of sort for fusion. So big clouds of hydrogen had to very slowly draw together via “molecular…cooling in the gas phase,” which meant more massive stars that burned furiously for a short while [1].
[1] https://iopscience.iop.org/article/10.1088/0004-637X/745/1/5...
Stars in distant galaxies are typically more massive than those in our "local neighborhood"
In particular, familiar stuff has ways to dispose of kinetic energy, allowing it to clump into our stars and planets, that might be unavailable to putative dark matter, making any dark matter more diffuse.
Generally, you can start with the assumption that people working on dark matter hypotheses are not idiots. They might still be wrong, but if they are, they will be wrong in interesting and subtle ways, not obvious ones.
Sure there is. It has similar interaction with regular matter. Any other properties need to be defined and shown to produce this claimed distribution as a result of a dynamic process. Anything less and we can just say dark matter is fairy dust.
Apparent effort to understand what you criticize may make your criticism seem worth more.
Actually no I cant. When someone looks at galaxy rotation curves and is surprised they dont follow keplers laws, it activates the bozo bit in my mind.
We still see younger galaxies with less massive stars not rotating as expected.
You mean younger, right?
Larger [More massive] stars are typically more luminous. Could be that there is a selection bias towards them. since they are the only ones that can be detected from that far away.
Does the mass and resultant gravitational force attenuate the same ratio for small vs big stars?
Also, how do super dense neutron stars work - in the sense that atoms (and neutrons) are “mostly empty space” — are the particles in the atoms compressed closer together? Or does an atom of one substance at the center of a star have the same mass/volume as an atom of the same, either floating by itself or say, on earths surface?
Here, obviously we're talking about stellar mass.
You can't really talk about the "volume" of an atom because the distance between one atom and the next closest atom really depends on the bulk material and isn't just atoms sorted into their one fixed size but is a balance of various forces.
A neutron star isn't made of atoms, but is, in a sense, one enormous atom or just a nucleus (but it's more complex than that). Mostly neutrons with some electrons and protons mixed in not separated into atoms which repel each other but in a super dense fluid of particles packed about as closely as particles in the nucleus of a normal atom.
Not only does earth have elements formed in older stars, so do all the planets in the solar system and our own sun.
We can never be certain, of course, because they are all long gone. Like, 13 billion years gone. The James Webb telescope is hoped to see back nearly that far back, but not likely near enough to catch any of them.