have we observed the universe expanding? is that even possible?

[Nuke]

supposedly the universe is bigger than we can see, but there is stuff so far away light hasn't had time to get here yet. however as time progresses forward, more and more light should in theory be able to reach us. so wouldnt we be able over time to see an unfolding horizon off in the distance? more stuff that was once behind the cosmic horizoncoming into view. or is some quirk of cosmic inflation preventing this observation?

Edited May 9, 2018 by Nuke

[HebaruSan]

Possible or not, it's been the consensus view among physicists for nearly a century now:

https://www.space.com/25179-hubble-constant.html

[p1t1o]

Theres a few factors in play here.

The most important is red-shift and blue-shift.

The next most important thing is that space itself is expanding, its more than just things getting "further" away from each other.

As all of space is expanding, it follows that the further away something is, the faster it  is moving away from us, and due to the fact that space itself is expanding, the furthest reaches can recede from us faster than light, this is what we call the edge of "the observable universe" as those parts whose light can never reach us are by definition not observable.

Back to red and blue shift. When light from a receding object reaches us, it does not arrive "slower" (see: relativity), light always arrives at c, but its frequency can shift.

The energy of EM radiation (light) is dependent on its frequency. 

When light from a receding object arrives, it arrives at c, but its energy will be lessened by a proportional amount to the difference in velocity between origin and destination. Thus, its frequency is lowered, moving it towards the red end of the spectrum (even if its nowhere near the visible portion of the spectrum, red-shift denotes a lowering of frequency, blue shift an increase). Yes, this does actually make some blue objects appear red.

It is very common for very distant objects to have their light red-shifted into non-visible parts of the spectrum (hence radio-telescopes are used to observe distant parts of the universe. Radio is the very-low energy part of the EM spectrum), this is one of the reasons why the night sky isnt a blaze of starlight from all directions.

Light from beyond the edge of the observable universe, where objects are receding from us faster than c, never reaches us. This seems to go against the idea that "light always arrives at c" but it might help to think of this light as being infinitely redshifted to invisibility.

Conversely, object approaching us quickly have their light blue-shifted. This is how we can tell that distant galaxies are rotating, because one side will be blue shifted and the other red shifted (or both red-shifted but to different degrees).

 

How can we tell if incoming light is blue or red shifted? Isnt it just light?

Yes, however light from hot things (stars and such) have close-to "black-body" spectra, in that the frequency of light emitted is very predictable and tells you what the material is that is glowing. Different elements show different patterns, appearing as spectral "lines" (a specific set of frequencies). When the pattern is red or blue shifted, the pattern can be seen to be in the "wrong" part of the spectrum, thus we can tell how blue or red shifted it is.

 

 

 

Edited May 9, 2018 by p1t1o

[kerbiloid]

Do the gravitational waves from the galaxies far, far away, suffer from the red shift, too?

Do they attract us weaker?
(Not just for r², but due to the energy loss.)

Does the universal gravity law require a red shift factor?

Edited May 9, 2018 by kerbiloid

[sevenperforce]

Yes, gravity waves are redshifted. But gravity waves are so weak when they reach us that minute frequency changes would be undetectable.

The key feature of the expanding universe is something called metric expansion. In other words, space itself is expanding, and everything is moving away from everything else, rather than everything moving away from some single point.

How do we know this? Well, suppose that the reverse was true, and everything was simply moving away from one specific region. If this was the case, then we would see redshift in most directions, but galaxies with the same vector as us (in other words, galaxies "behind" or "in front" of us, relative to the region everything is moving away from) wouldn't be redshifted at all. Or, depending on how the expansion was happening, some would be redshifted and some would be blueshifted. 

Instead, we see redshift in all directions, and the farther away something is, the more redshifted it is. The only way this is possible is if everyone everywhere in the universe is seeing the same thing as everything is moving away from everything else.

2 hours ago, Nuke said:

supposedly the universe is bigger than we can see, but there is stuff so far away light hasn't had time to get here yet. however as time progresses forward, more and more light should in theory be able to reach us. so wouldnt we be able over time to see an unfolding horizon off in the distance? more stuff that was once behind the cosmic horizoncoming into view. or is some quirk of cosmic inflation preventing this observation?

We are fortunate to live in a (cosmologically) brief period of time during which we can still see the afterglow of the big bang. Before the big bang, the whole universe was in a hot, dense state, and then inflation started. After inflation, the universe was not nearly quite so hot or dense...but was still about as hot and dense as the core of the sun, which is pretty hot and pretty dense.

The universe continued to expand in the post-inflationary epoch, for about three hundred centuries. At this point, it had expanded and cooled to the approximate density and temperature of the surface of the sun. At this point, however, light still didn't exist as we know it. The whole universe was full of plasma, and with nowhere to go, photons simply didn't propagate between atoms. It was all one big dark hydrogen ocean.

Then, at three hundred centuries of age, the expansion broke the light and the darkness apart, and photons decoupled from hydrogen atoms and scattered out into the new universe. The whole universe changed from dark to light in a single moment.

Nearly 14,000,000,000 years later, we can still see those photons from last cosmic scattering. We sent up satellites to take a picture of it all. Here's what it looks like:

So there you have it.

 

[kerbiloid]

5 minutes ago, sevenperforce said:

But gravity waves are so weak when they reach us that minute frequency changes would be undetectable.

But this is not a question of detectabilty.
This a question of the gravity force equation.
There were numerous attempts to insert an exponential factor, or add a very small alpha to the square degree, but they always said NO! and rejected this.
But if the redshifted light heats us weaker, doesn't it mean that the gravity equation is not just gMm/r² ?

[sevenperforce]

18 minutes ago, kerbiloid said:

But this is not a question of detectabilty.
This a question of the gravity force equation.
There were numerous attempts to insert an exponential factor, or add a very small alpha to the square degree, but they always said NO! and rejected this.
But if the redshifted light heats us weaker, doesn't it mean that the gravity equation is not just gMm/r² ?

Ah, I see what you're asking.

The answer is no. Redshift of gravitational waves does not change the parameters of the gravitational field tensor. You just have to solve the tensor equations for an expanding reference frame.

Also, redshift of gravity waves doesn't mean the gravity waves are weaker. Gravitational force is proportional to gravitational potential, represented by the amplitude of the gravitational wave. Redshift is a change in the frequency of a wave.

Redshift lowers the frequency (and the corresponding flux energy) of photons coming from distant galaxies. In that sense, redshifted light is "weaker". But this is to be expected; the photon has been stretched across a greater amount of space, and so that energy is spread out. If a star emitted a photon when it was 1 billion lightyears from us, and that star is now 2 billion lightyears from us, then the photon has been stretched out to twice its original wavelength or half its original frequency.

[wumpus]

1 hour ago, sevenperforce said:

Ah, I see what you're asking.

The answer is no. Redshift of gravitational waves does not change the parameters of the gravitational field tensor. You just have to solve the tensor equations for an expanding reference frame.

Also, redshift of gravity waves doesn't mean the gravity waves are weaker. Gravitational force is proportional to gravitational potential, represented by the amplitude of the gravitational wave. Redshift is a change in the frequency of a wave.

Redshift lowers the frequency (and the corresponding flux energy) of photons coming from distant galaxies. In that sense, redshifted light is "weaker". But this is to be expected; the photon has been stretched across a greater amount of space, and so that energy is spread out. If a star emitted a photon when it was 1 billion lightyears from us, and that star is now 2 billion lightyears from us, then the photon has been stretched out to twice its original wavelength or half its original frequency.

First of all, computing the Fourier transform for what is detected in a gravity wave is going to be pretty sketchy (have some pretty big error bars).  I can't imagine trying to compute the "expected frequencies from said gravity wave": it would presumably require multiple sources of data on what event caused the gravity wave (presumably two black holes slamming together, but why that frequency?).  Hubble did his research in the 1920s using light from observed stars (obviously easily available using 17th century methods).  I think something like 2 or so gravity waves have been detected, it is far to early to know what a "gravity redshift" is supposed to look like.

[sevenperforce]

7 minutes ago, wumpus said:

First of all, computing the Fourier transform for what is detected in a gravity wave is going to be pretty sketchy (have some pretty big error bars).  I can't imagine trying to compute the "expected frequencies from said gravity wave": it would presumably require multiple sources of data on what event caused the gravity wave (presumably two black holes slamming together, but why that frequency?).  Hubble did his research in the 1920s using light from observed stars (obviously easily available using 17th century methods).  I think something like 2 or so gravity waves have been detected, it is far to early to know what a "gravity redshift" is supposed to look like.

The redshift of a gravitational wave would be shown as an increase in the length of the measured duration of the gravitational merger event.

The last LIGO detection event lasted 0.15 seconds, I believe. The merger itself took much longer, of course; that 0.15 seconds was only the length of time during which the amplitude of the spacetime fluctuation exceeded LIGO's detection limits. But that portion of the merger was originally lower.

Redshift at the edge of the universe is a wavelength multiplication factor of around 3.4; if LIGO detected this black hole collision at the edge of the universe, then the true duration (of this portion of the spacetime fluctuation) was probably something like 0.04 seconds and it has simply been stretched out. But the galaxy with the black hole merger was 1 billion lightyears away. That's far, but it's not very far in cosmological terms; only about 2% of the way to the edge of the observable universe. And expansion is metric. So the actual duration of this part of the fluctuation was probably just 0.14999999999995 seconds or so.

Come to think of it, that's a theoretical (if experimentally impossible) verification of Hubble's Law. Since the masses of the black holes can be known independently, their merger could be modeled, and the duration of the merger peak could be predicted. Deviations in the duration of the merger peak would be a direct measure of redshift.

[mikegarrison]

I have been expanding as I get older. Perhaps the universe is the same.

[sevenperforce]

46 minutes ago, mikegarrison said:

I have been expanding as I get older. Perhaps the universe is the same.

And like the universe, you expand fastest when you are youngest.

Almost...inflationary.

[kerbiloid]

7 hours ago, mikegarrison said:

I have been expanding as I get older. Perhaps the universe is the same.

You do this together.

So, relative to the Universe, you stay thin.