I have a science question floating around in my head, and I am hopeful somebody here can provide some insight.

Hubble's Law, galaxies red shifting, everything is expanding. I'm good with that. However, I feel as though an incredibly important variable has been left out in arriving at the conclusion that the universe is expanding at an accelerating rate based on distance. And that variable is time.

I have heard many times that looking into space is like looking back into time. This, of course, is because it takes time for light to travel. Inevitably, when Hubble looked at the light from galaxies of varying distances, he was also looking at light from varying times. Let's call the amount of time the light has taken to reach Earth the "age" of the light.

To help illustrate what I am trying to say, lets look at a graph of Hubble's Law.



What if the x-axis was converted from distance in mega-parsec to "age" of the light in years? We now have a graph strongly correlating the velocity of recession with the "age" of the light. The older the light, the faster it is (was?) moving.

So which question is Hubble's Law actually answering:

The velocity at which galaxies move based on how far away they are?

Or

The velocity at which galaxies moved based on how far ago they are?

When we study a fossil from 100 million years ago, we do not conclude that it is a representation of life on Earth in the present. Instead we say, "Wow look at what life on earth used to be like 100 million years ago." The fossils are a portal into the past. So is space light. Instead of looking at 13 billion year old light and saying look at how fast the universe is expanding, should we not, instead, be saying look at how fast the universe was expanding 13 billion years ago?

If viewing the graph in the way I described (essentially a timeline), it paints the picture of a universe that is expanding at a decelerating rate. This also makes more logical sense to me. Big bang, huge amounts of energy being converted into matter, this matter flying off in every direction at astronomical speeds, gravity over billions of years tugging on all this matter trying to bring it back together, matter still getting further apart, but decelerating, and maybe eventually reversing course.

Of course, the universe is under no obligation to make logical sense to me, and the human brain did not evolve with selective pressures on understanding how the universe works.

So I guess my question is:

Does adding time to this problem have any merit?

If so, what is the impact of this? If not, why is time not an integral component of Hubble's Law?

Thanks in advance to anyone who can provide clarity on this for me. I have been searching the internet and found nothing regarding time and its impact on Hubble's Law.

I'm not an astronomer or anything, but I think the problem is that you are conflating two different things. The light that left distant galaxies began its journey 13 billion years ago, but the red-shift happened during the light's travel to us.

Yes, the expansion could have stopped or slowed and we wouldn't know it, but I don't think it's a problem to say "is" expanding since that is all the information we could ever have according to the laws of physics.

I also read that observations show that until (edit: ) 5 billion years ago the universe's expansion was slowing down. Maybe reading up on how this was determined will give you some insight.

This thread may shed some light on your question if you have the time . . .

https://forumserver.twoplustwo.com/4...oblem-1652550/

The galaxies them selves are not moving away from us through space. It is the actual fabric of space which is expanding. A larger recessional velocity in this context simply means a larger amount of space expansion. The amount of redshift seen is therfore directly related to, but not explicitly related to the amount of expansion of space between us and the galaxy inbetween the time of emission of the light and the time of observing the light. It therfore doesent give us any indication of the initial recessional velocity of the galaxy.

Fabric of space?

Chambray, I believe.

Which ever is softest when it counts.

It's a relativley common analogy used when representing space as a 2d grid, more like your grans cross stitch knitting than your favorite brand of cleansing paper

https://forumserver.twoplustwo.com/4...53/index3.html

Imagine that at t=0, you are at a distance of 1 sound-second from an ambulance that is moving away from you at half the speed of sound. At t=1 you hear the sound transmitted at t=0. But the ambulance is now 1.5 sound-seconds away, so you will only hear the sound transmitted then at t=2.5. As such you can only determine what the ambulance was doing at the time of transmission, 1+t/2 seconds ago.

However, if at t=1 you yourself move away from the position of the ambulance at half the speed of sound, that sound transmitted at t=1 will now reach you at t=4. This additional Doppler effect is caused by your speed at the time of observation.

As such, the total Doppler effect is a combination of how fast the object was moving away from your current position at the time of transmission and your current speed away from the position of the object at the time of transmission.

Please note that I was talking about the speed compared to the position of an object at a certain time and not the object itself. For instance, while at t=2 your relative speed to the ambulance is the speed of sound, your speed away from the position where the ambulance was at t=2 is half the speed of sound.

In conclusion: The red shift of distant galaxies depends both on the speed in the past and the speed in the present.

Okay, that was very hard to comprehend. Let's try again. Consider these signals:



The orange signal is transmitted by the moving galaxy. The red signal is how that signal would be observed in a (hypothetical) stationary point in the universe. The difference between the red and orange signals is caused by the speed of the transmitter at the time of transmission.

The blue signal is how the signal would be observed by somebody moving in the opposite direction. The difference between the blue and red signals is caused by the speed of the observer at the time of observation.

As such the difference between the known orange signal and the observed blue signal is caused both by the speed at the time of transmission and the speed at the time of observation.