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The universe is expanding faster than expected

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Re: The universe is expanding faster than expected

#211

Post by bmcn2020 »

vecter wrote: post #176
johnnujler wrote: post #172

Forgive my ignorance here. What do we mean when we say the universe is expanding, expanding into what. My untrained brain is trying to look at it like the inflation of a balloon that takes up more space when it is inflated, but if the universe is all the space there is, what would it even mean for it to expand. Is there space outside our universe that is like unfilled or something?

Your balloon analogy is almost correct but it's missing one important piece. In that analogy, the surface of the balloon itself is the entire universe. It is a two-dimensional universe. Imagine that there is no other space except for that surface of the balloon. No emptiness inside and no space outside that it's "expanding into". The entire space is the infinitesimally thin surface which is the balloon surface, yet i…

i guess my question would be about the analogy

in these analogies, the balloon surface is made of rubber, or the stretchy thing is made of whatever stretchy things are made of

is space something tangible? what is it made of? what is its physical property? how can it stretch?

Re: The universe is expanding faster than expected

#212

Post by tokamak-teapot »

throwaway201103 wrote: post #182
bouncycastle wrote: post #166

Earlier quoted context omitted.

However, after the universe finishes expanding, wouldn't it start contracting again? Btw, Andromeda will collide with the Milky Way in about 4 billion years, so at least we won't need ten times the mass of Mt. Everest to get there...

If you subscribe to Roger Penrose's theory, at the point of infinite expansion, the current "Aeon" will end and a new big bang will happen. As best I understand it, at the point of infinite expansion, there are only photons. Mass no longer exists therefore scale no longer exists. So an infinitely huge universe becomes indistinguishable from an infinitely small universe.

What causes the Big Bang in this scenario?

Re: The universe is expanding faster than expected

#213

Post by nathanyukai »

bmcn2020 wrote: post #211
vecter wrote: post #176

Earlier quoted context omitted.

Your balloon analogy is almost correct but it's missing one important piece. In that analogy, the surface of the balloon itself is the entire universe. It is a two-dimensional universe. Imagine that there is no other space except for that surface of the balloon. No emptiness inside and no space outside that it's "expanding into". The entire space is the infinitesimally thin surface which is the balloon surface, yet i…

i guess my question would be about the analogy in these analogies, the balloon surface is made of rubber, or the stretchy thing is made of whatever stretchy things are made of is space something tangible? what is it made of? what is its physical property? how can it stretch?

Nobody knows. Dark matter and Dark energy probably, but again no one knows what they are either :)

Re: The universe is expanding faster than expected

#214

Post by cygx »

bmcn2020 wrote: post #211
vecter wrote: post #176

Earlier quoted context omitted.

Your balloon analogy is almost correct but it's missing one important piece. In that analogy, the surface of the balloon itself is the entire universe. It is a two-dimensional universe. Imagine that there is no other space except for that surface of the balloon. No emptiness inside and no space outside that it's "expanding into". The entire space is the infinitesimally thin surface which is the balloon surface, yet i…

i guess my question would be about the analogy in these analogies, the balloon surface is made of rubber, or the stretchy thing is made of whatever stretchy things are made of is space something tangible? what is it made of? what is its physical property? how can it stretch?

That's where the analogy breaks down. If you take Einstein relativity at face value, there's no priviledged spatial slicing (contrasting with, say, Lorentz ether theory), and the balloon is not really a thing.

The most obvious interpretation of general relativity is in terms of B-theory of time: Spacetime is some 'pre-existing', 'eternal' thing over mich matter is distributed. This also fixes the geometry (things like lengths and angles) via Einstein's equations, which more or less state that energy-momentum ∝ Ricci curvature.

In our universe, that distribution comes in layers, ie there's a spatial slicing where the matter distribution appears homogeneous. In that sense, there is a priviledged slicing, which has the unfortunate side effect of making people forget the lessons of special relativity.

Now, the average density of matter changes from layer to layer, and, if our universe were described by the 'closed' Friedmann model, so would the (finite!) volume of the slice. That change is not arbitrary: The layers can be labelled by cosmological time, and with its increase, the average proper distance between galaxies increases as well. That's called the metric expansion of space, because in our idealized model, the metric (a thing defining distances and angles) within a given slice is just a scaled version of the one in a different slice.

Re: The universe is expanding faster than expected

#215

Post by ricardo81 »

cma wrote: post #117
ricardo81 wrote: post #2

One thing I've always admired as a Layman about astronomy, is all the eloquent deductions they've been able to make that adds another framework of ways to discover new information, particularly the discovery of Cepheids. Obviously the engineering side is also as impressive, being able to look at objects light years away and using the paltry several hundred million miles our Earth orbit takes us around the sun as a me…

Good overview here: https://en.wikipedia.org/wiki/Cosmic_distance_ladder More: https://www.math.ucla.edu/~tao/preprints/Slides/Cosmic%20Dis... https://terrytao.wordpress.com/2020/10/10/climbing-the-cosmi...

Really enjoyed the math.ucla.edu link, good pace for me. Thanks.

Re: The universe is expanding faster than expected

#216

Post by aliceryhl »

bmcn2020 wrote: post #211
vecter wrote: post #176

Earlier quoted context omitted.

Your balloon analogy is almost correct but it's missing one important piece. In that analogy, the surface of the balloon itself is the entire universe. It is a two-dimensional universe. Imagine that there is no other space except for that surface of the balloon. No emptiness inside and no space outside that it's "expanding into". The entire space is the infinitesimally thin surface which is the balloon surface, yet i…

i guess my question would be about the analogy in these analogies, the balloon surface is made of rubber, or the stretchy thing is made of whatever stretchy things are made of is space something tangible? what is it made of? what is its physical property? how can it stretch?

No, space is not something tangible. It is not really made of anything.

Re: The universe is expanding faster than expected

#217

Post by xwdv »

rytill wrote: post #20
xwdv wrote: post #13

Earlier quoted context omitted.

The worry is that not only will we be utterly destroyed, but that the destruction will be so thorough it will blow backward through time erasing all events that ever happened and making it so that we never even lived. That means everything we experience right now didn’t happen, we are just seeing a probability of what could happen but didn’t because it’s all destroyed. These lives mean nothing.

I don't see how it's possible to un-make-something-happen. It already happened. It's gone. You can't kill what's already dead.

Imagine they told you that when you die, not only will you be gone, but then they will go back and undo everything you ever did and basically make it like you never existed at all. It didn’t happen, you never happened.

Re: The universe is expanding faster than expected

#218

Post by Panoramix »

User23 wrote: post #80
lisper wrote: post #72

Earlier quoted context omitted.

> why don't you please present me with both? If you are not a YEC, then the first thing I'm going to do is ask you why you think that cosmology and physics are different? If you are a YEC then I don't need to ask you that question because I already know the answer. (It's because you have accepted the truth of the Bible as a foundational assumption. From this, the idea that modern cosmology is not science is a logical…

If I were not a YEC, then I'd think cosmology and physics are different because physics is a laboratory science where depending on our budget we can do anything from roll balls down inclines to produce quarks while with cosmology we have absolutely no control over conditions and can only make believe we do by sampling in a way that necessarily is corrupted by our biases. And the amount of data to sample from is plent…

I'm sorry to say your understanding of physics is pretty wrong.

For both, we control almost nothing, we observe. To that end we build tools to observe. We want to observe to reject hypotheses or to get new insight on how nature behaves. In particle physics these are detectors, like the Super Kamiokande [1], which is just sitting there waiting for neutrinos to arrive from space. In the case of cosmology these are telescopes, radio telescopes and the like, waiting for photons, gravitational wave chirps to arrive from space.

[1] https://en.wikipedia.org/wiki/Super-Kamiokande

Re: The universe is expanding faster than expected

#219

Post by gpderetta »

pdonis wrote: post #177
gpderetta wrote: post #139

Earlier quoted context omitted.

Energy is not conserved at those scales because of general relativity, which also provides the mechanism for an expannding universe. Einstein himself didn't like this result of GE and tried to hack around it with the cosmological constant. Turns out the math was right.

> Turns out the math was right. Actually, turns out the math was right with a cosmological constant: that's how the accelerating expansion of our universe is explained. Einstein actually blundered twice : first by putting in the constant for the wrong reason (because he wanted a static universe), but then taking it out again when it turned out that reason wasn't the case (when the expansion of the universe was discov…

Re: accellerating expansion, I was only commenting on how the expansion itself is a consequence of GE.

Thanks for your point about the constant being actually required. I do not understand the math, but is this similar on how integrals always have a constant as a free parameter that need to be determined by other means?

I assume that Einstein originally set the constant to exactly balance the expansion, but later set it to zero. In bot cases you are picking arbitrar values but the actual value need to determined by empirical observeations.

Re: The universe is expanding faster than expected

#220

Post by azernik »

pratyushc wrote: post #198
azernik wrote: post #130

Earlier quoted context omitted.

Another interesting one I've heard is getting into space, period. It's possible that there are other technological civilizations out there, but all stuck under a few hundred km of ice or on a 2-earth-mass monster where it's impractical to reach orbit. Earth may only be special in being in a sweet spot between Mars (too small to hold an atmosphere or protect from cosmic radiation, hence no life) and Gliese 832c (with…

> protect from cosmic radiation That raises another issue of the planet having a magnetic core...

You can also get shielding with a thick enough atmosphere, or being under a solid ice/rock crust. But both of those also make it much harder to reach space.
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