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Here's what I just read from Wikipedia's page on the Hubble Space Telescope:

While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and the Supernova Cosmology Project used ground-based telescopes and HST to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. The cause of this acceleration remains poorly understood; the most common cause attributed is dark energy.

I don't take this at face value because we should expect more distant objects to have higher observed speeds and therefore higher observed red-shifts. Here's why.

  1. Let's start with a model where the Universe expanded very fast early on, but has been slowing down ever since due to gravity, as one would normally expect.

  2. Remember that, the farther away a cosmic object is, the farther back in the past we are observing it. An object 1,000 light years away, if it's light is reaching us now, is being observed in its state that existed 1,000 years ago. We are effectively looking through a time machine.

  3. So if we observe a more distant object, we're observing an older state of that object. Therefore, we are observing it at a time when the Universe was expanding faster than it is now, so it has higher red-shifts.

And isn't that what we observe today? The more distant the galaxy, the higher its red-shift? This is not inconsistent with a "normal" model where the expansion is slowing down due to gravity.

What am I missing? Why are scientists trying to explain such things with weird dark matter and dark energy that otherwise have never been detected or found evidence of and aren't needed for any other model, and in fact get in the way of our models of physics and quantum dynamics?

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  • $\begingroup$ they took multiple measurements and found that the later ones indicated a higher recession velocity. $\endgroup$ – Astrony Jun 13 '15 at 7:10
  • $\begingroup$ @Astrony can you provide any numbers or a citation for that? If it was only a few years timespan between measurements, that seems like a pretty short time on the scale of the Universe and I can't imagine that our instruments would pick up any noticeable change. $\endgroup$ – DrZ214 Jun 13 '15 at 7:20
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    $\begingroup$ That Wikipedia article is not saying what you seem to be thinking it says. The SN results are not consistent with a constant Hubble constant and are consistent with models with the Hubble constant increasing with time. If you are interested in the detail see here $\endgroup$ – Conrad Turner Jun 13 '15 at 10:36
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    $\begingroup$ The yeast becomes more active over time. Eventually the universe will be over proofed. $\endgroup$ – Wayfaring Stranger Jul 15 '15 at 12:34
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I don't take this at face value because we should expect more distant objects to have higher observed speeds and therefore higher observed red-shifts.

That's true. That was the original Hubble discovery - the farther away things were, the faster they were moving away from us.

Here's why. Let's start with a model where the Universe expanded very fast early on, but has been slowing down ever since due to gravity, as one would normally expect.

Yes - that's what everybody thought following Hubble's discovery.

Remember that, the farther away a cosmic object is, the farther back in the past we are observing it. An object 1,000 light years away, if it's light is reaching us now, is being observed in its state that existed 1,000 years ago. We are effectively looking through a time machine.

This is not lost on Astrophysicists.

So if we observe a more distant object, we're observing an older state of that object. Therefore, we are observing it at a time when the Universe was expanding faster than it is now, so it has higher red-shifts.

OK, 2 points. 2nd point first. The red-shift has to do with relative velocity, not speeding up or slowing down. Something can be more red-shifted and slowing down and something can be less red-shifted and speeding up, especially since the acceleration/deceleration is comparatively slow compared to the relative velocity.

and other point - lets keep in mind, we don't know what a galaxy 3 billion light years away is doing now. We can guess and we can run models, but we can only see what it's doing 3 billion years ago.

And isn't that what we observe today? The more distant the galaxy, the higher its red-shift? This is not inconsistent with a "normal" model where the expansion is slowing down due to gravity.

Yes, the more distant the galaxy the higher it's red-shift. But no, that's not inconsistent with expansion. That's what you'd see, expansion or contraction, because red-shift is just relative velocity.

What am I missing? Why are scientists trying to explain such things with weird dark matter and dark energy that otherwise have never been detected or found evidence of and aren't needed for any other model, and in fact get in the way of our models of physics and quantum dynamics?

A lot of these ideas are confusing. They're confusing to scientists too, especially when they were first discovered - so you're not alone.

Dark matter was observed because galaxies were behaving strangely. The stars in the outer arms of the galaxy were observed to be moving much too fast and faster than the stars more towards the middle of the galaxy and that made no sense. The galaxies also weighed too much and the only way to explain this was extra mass in kind of a halo around the galaxy, but this extra mass, also, didn't interact with electromagnetic waves like the mass here on earth does - so they called this extra mass (and there's a lot of it, more than there is regular mass), but since it's invisible, they called it "dark matter" and it's not dark like dirt or coal, it's dark as in - invisible. It's completely transparent to light, but it has mass and they still don't know what it is. They have some OK theories, but nothing definite.

Now, dark energy - think about the big bang and all matter flying apart - the galaxies twice as far are moving away twice as fast, BUT, as you said, because of gravity, we should see the galaxies that are twice as far moving away more than twice as fast, cause the nearer the galaxy, the more time it's had to slow down - aha, they thought, if we can compare the speed of the galaxies 4 billion light years away to the speed of the galaxies 2 billion light years away to the speed 1 billion light years - etc, etc and measure it all carefully, we can measure the rate at which gravity is slowing down the universe. - that makes sense right.

And with careful measurement of Type 1A supernovas, which temporarily outshine entire galaxies - with remarkable consistency (what they call a standard candle - a very bright standard candle, but a standard candle all the same) - with that, they thought they could measure the gravitational slow down of expansion - exactly what you're talking about.

The problem was, the measurements told them the opposite of what they expected to find. The measurements told them that the galaxies 2 billion light years away were traveling slightly more than half as fast as the galaxies 4 billion light years away, and so on. They checked this, cause it had to be wrong, then they re-checked it, and re-checked again and the only conclusion was, stuff out there is speeding up, not slowing down - cause that's what the telescopes tell us.

Dark energy wasn't a hair-brained scheme that mad scientists thunk up. It was an observed reality that nobody expected (well, cept just maybe for Einstein and his cosmological constant, but that's another story).

Dark energy's just a name anyway. They have to call it something, even if they're not sure what it is or how it works.

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  • $\begingroup$ This starts to explain it. I was actually thinking a similar thing for the discrepancy in orbital speeds of certain parts of the galaxy, and was hoping someone would mention it, but i thought i'd better pick either that or the expanding universe so i don't get closed for being too broad. However, when you measure something like a Type 1a supernova and get unexpected results, doesn't that imply that maybe something is wrong with your stellar model? Or the model of Type 1a supernovas? Also, how do you know the distance to a galaxy? You have to assume its absolute magnitude first, don't you? $\endgroup$ – DrZ214 Jun 13 '15 at 23:14
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    $\begingroup$ From WIki: en.wikipedia.org/wiki/Type_Ia_supernova "The typical visual absolute magnitude of Type Ia supernovae is Mv = −19.3 (about 5 billion times brighter than the Sun), with little variation" It is their absolute magnitude that made the measurement's possible. $\endgroup$ – userLTK Jun 13 '15 at 23:34
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    $\begingroup$ There is some variety in the luminosity (+/-1.5%), but combined with the decline rate, they think they have a pretty good standard candle. - source: astronomy.swin.edu.au/cosmos/T/Type+Ia+supernova $\endgroup$ – userLTK Jun 14 '15 at 0:18
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According to this paper I understood that scientists have found that redshift (z) of supernovae depends on relative distance (x) according to formula: z = 2 sinh(x)/(1 + x). And IF nothing is pushing galaxies away with force then redshift should drop linearly with decrease of distance between galaxies. But it isn't so as relationship z(x) is a nonlinear one.

Lets draw a graph: enter image description here In other words - when distance to a supernova drops in half, the redshift must drop in half too (i.e. linearly), if gravity is stopping expansion. But it isn't the case as can be seen in the graph. The only reasonable explanation: that there exists some force which doesn't let that happen and keeps increasing the expansion rate.

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When they look at the light from distant galaxies, they see peaks and troughs.

The peaks and troughs are from the spectra of the atoms in the galaxies - for instance hydrogen, oxygen, etc.

From frequency and the patterns of emission and absorption of the light - or the peaks and troughs, they can identify the element.

When they compare the spectra of an elements from the galaxies to the spectra of the elements in the lab, they find they're shifted to the red - or red shifted.

The point is, they are not measuring velocity - they are inferring velocity.

To infer velocity measurements from the spectra, one has to assume the processes which created the spectra in the star billions of years ago is identical to the processes which created the spectra in the lab.

And they have to assume there are no non-velocity red shifts.

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  • $\begingroup$ Hi Cinaed, welcome to our site. This is a good basic explanation of redshifts, with an insightful comment about assumptions thrown in - but I'm not sure it answers the original question. Perhaps you might edit your post to respond to the question title and the OP's last paragraph? For further guidance, see How to Answer, and don't forget to take the site Tour. :-) $\endgroup$ – Chappo Says Reinstate Monica Mar 21 at 4:55

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