

The Two Big Bangs - 127001brewer
https://medium.com/starts-with-a-bang/the-two-big-bangs-1493194f5cd9

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dak1
This is probably a rather simplistic question to answer, but I've been
confused how we go from "on average, the farther away a galaxy is, the faster
it’s moving away from us" to "the expanding Universe".

If every galaxy was observed in its present day state, then that conclusion
would seem to be valid. However, when we're looking at a galaxy 5 billion
light years from us, we're looking at it as it was 5 billion years ago
(including the light shift).

Therefore, it seems at least plausible that expansion is slowing rather than
accelerating, since galaxies were moving away faster the further back you go
in time.

I'm sure the above can be explained, although it may require a lot more
time/effort to refute than it took for me to make the argument, but could
somebody with more training in the subject point out the flaw in my logic or
explain why the expansionist interpretation fits better with observation?

~~~
Strilanc
The history of astronomy is filled with people figuring out how to measure
apparently-impossible-to-measure things. For example, how would you go about
measuring the distance to the sun? (Keep in mind that you're not allowed to
use things we derived _from_ that measurement and take for granted now.)

When you look at a galaxy from 5 billion years ago, you're _not_ looking at it
as it was including the light shift. The red-shift they're talking about is
not from the initial difference in velocity, it's from the wave length being
stretched by the expansion of space as the light travels.

For example, the wikipedia article about the accelerating expansion [1]
mentions things like:

> the distance-redshift relation deviates from linearity, and this deviation
> depends on how the expansion rate has changed over time

If the universe decided to suddenly increase in size by 1% over the next hour,
all the already-travelling light would arrive with a 1% larger wavelength. If
it had suddenly increased in size by 1% a billion years ago, the already-
travelling light would get just that 1% larger wavelength but also 1% more
travel time for stretching out to occur over. So if you have an independent
way of determining distance... a lot of information falls out of that red-
shift-vs-distance plot.

1:
[http://en.wikipedia.org/wiki/Accelerating_universe](http://en.wikipedia.org/wiki/Accelerating_universe)

~~~
WA
_For example, how would you go about measuring the distance to the sun?_

So, what's the answer? Where can I read more about that?

~~~
rikkus
A clever chap figured out a method a couple of thousand years ago:
[http://en.wikipedia.org/wiki/Aristarchus_of_Samos](http://en.wikipedia.org/wiki/Aristarchus_of_Samos)

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semaphoreP
I wonder if this result that there was no singularity true for all theories of
inflation or just a subset of them.

~~~
pdonis
What the article is referring to is that de Sitter spacetime, which is an
idealized universe that only contains vacuum energy, has no initial
singularity; it extends infinitely far back in time (and infinitely far
forward as well). This is the basic model (at the classical level--see below)
used to describe inflation, so in that sense I think the answer is that the
"no singularity" result applies to all inflation theories.

That's not quite the end of the story, because the idealized model I just
referred to is a classical model; it doesn't include quantum effects. I think
the general opinion among physicists is that, if we extrapolate inflation back
far enough, we reach a point where quantum effects become important, and the
classical model above no longer applies. I don't think anyone expects a
quantum model with an initial singularity to replace the classical model in
this regime; but at this point we don't know what the correct quantum model
is.

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tjradcliffe
Since the very earliest days of Big Bang cosmology there has been a debate
over the initial singularity, because nobody likes singularities.

There was a debate about black holes for the same reason. The original Thorne-
Hawking bet (scroll down past the more recent one on black hole information:
[https://en.wikipedia.org/wiki/Thorne%E2%80%93Hawking%E2%80%9...](https://en.wikipedia.org/wiki/Thorne%E2%80%93Hawking%E2%80%93Preskill_bet))
was driven in part by discomfort with singularities. There was and is a
feeling that _something_ ought to make them go away, but we remain unsure if
they really will, and until we have a genuine theory of quantum gravity our
accounts of what happens at very high densities is going to be a bit hand-
wavy.

So this article's suggestion that the current situation is particularly new or
different isn't all that accurate. There have always been two senses of the
meaning of "the Big Bang" and the degree of comfort people have had with
singularities has varied over time (lower in the '60's and '70's, higher in
the '80's and '90's, lower again today.)

Nor is it accurate to say that inflationary cosmology is completely
uncontroversial and so well understood that we have no choice but to discard
the initial singularity. Inflationary cosmologies are popular for good
reasons, but they are not the only game in town--we recently saw a thing on
Bohmian mechanics go by on HN that suggested an alternative to inflation, for
example (I'm far from sold on the work, but use it simply as a way of pointing
out that alternatives to inflation exist and can't quite be discarded
completely just yet.)

