
The universe has made almost all the stars it will ever make - dnetesn
http://nautil.us/issue/89/the-dark-side/the-universe-has-made-almost-all-the-stars-it-will-ever-make
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pdonis
I think the article headline is somewhat misstating the conclusion of the
actual paper (linked to by alex_young). The actual paper says the stellar mass
density will reach a maximum only 5% higher than its current value. But the
stellar mass density is affected by two processes: star formation _and_ star
destruction. As the paper notes, the process of star destruction "recycles"
some fraction of the mass of the destroyed star, making it available for new
star formation. So the stellar mass density reaching a maximum does not mean
that all star formation stops, which is what the article headline implies.

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causality0
I wish there was a way to flag posts as "appropriate for HN but has a wildly
misleading headline".

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m463
I would imagine some kind of a/b testing algorithms have come up with "wildly
misleading headline" => more interest.

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eru
If you assume that interestingness of headlines is somewhat randomly
distributed, and that for each article there are more misleading headlines
than accurate headlines, your relation is bound to be true, even if being
misleading by itself doesn't help.

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alex_young
This appears to be the paper actually stating the headline [0].

Interestingly the statement is a bit different: "The star formation activity
over the last ∼11 Gyr is responsible for producing ∼95 per cent of the total
stellar mass density observed locally, with half of that being assembled in 2
Gyr between z = 1.2 and 2.2, and the other half in 8 Gyr (since z < 1.2). If
the star formation rate density continues to decline with time in the same way
as seen in the past ∼11 Gyr, then the stellar mass density of the Universe
will reach a maximum which is only 5 per cent higher than the present-day
value."

To me, this says a lot more about the expansion of the universe, in that we
have an ever smaller window into the observable universe, than anything about
black holes.

Is that a reasonable interpretation or am I missing something?

[0]
[https://academic.oup.com/mnras/article/428/2/1128/1000290](https://academic.oup.com/mnras/article/428/2/1128/1000290)

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pdonis
_> To me, this says a lot more about the expansion of the universe, in that we
have an ever smaller window into the observable universe_

I don't think that's what the actual paper is saying. The "stellar mass
density" it talks about is by definition homogeneous across the entire
universe; it's not something that is affected by how much of the entire
universe we can actually see.

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alex_young
“Observed locally” seems to be doing a lot of heavy lifting if I read things
right.

If the observable universe is say 1/300th of the total, wouldn’t this actually
represent an increase?

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pdonis
We can only make direct observations of stellar mass density locally. We have
no direct way of knowing what the stellar mass density is "now" a billion
light years away; all we can see is what it was a billion years ago. We have
to reconstruct what things are like "now" elsewhere based on past data, which
is all we can see.

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alex_young
That makes sense, but isn’t it at least more likely than not that there is
relatively normal distribution?

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pdonis
_> isn’t it at least more likely than not that there is relatively normal
distribution?_

No, because that would make our particular spatial location in the universe
special, and that is an additional hypothesis that would require some kind of
evidence in its favor, and we have none. The simplest hypothesis is that the
universe is pretty much the same everywhere, and that means the average
stellar density "now" should be pretty much the same everywhere.

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alex_young
That’s what I meant by normal distribution :)

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pdonis
Ah, ok. I was interpreting "normal distribution" in the sense that term is
used in statistics, as "Gaussian centered on our spatial location".

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prawn
_" Research over the past 30 or so years has revealed that the formation of
stars across the universe reached an extended peak of activity roughly 10 to
11 billion years ago."_

 _"... it appears that the great majority of stars that the universe will ever
make—perhaps 95 percent of them—have already been made."_

Not sure it matters on human timescales, but still feels like we're late to
the party.

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jodrellblank
Since heavier elements are made by stars, could we have been much earlier? How
much earlier?

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andi999
Good question. I remember that somehow we hafäd to go through at least 3 star
creation/destruction cycles to get all our elements

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Tepix
Large stars have a short lifespan

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maest
Related question:

At universe-scale, does it makes sense to talk about a general "now"? I
thought that, when large enough distances are involved, "now" becomes a local
concept.

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contravariant
It gets more complicated but it still makes sense. The concept is called a
Cauchy surface.

The tricky bit is that it isn't uniquely defined, but even for an expanding
universe it can be defined globally (I think).

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cheez
The data points to this but "Einsteins greatest blunder" and earth-centric
universe come to mind.

Simply (and I'm sure many have thought of alternatives), it is based on what
we see. If, as is commonly thought, the universe will expand till we are
alone, then there is no guarantee that this hasn't happened already which
means we may have no idea what lies outside the cosmological horizon.

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yters
In a certain sense, everything has existed since the moment of the big bang,
since all the information necessary to define the universe's evolutionary
trajectory needs to have existed since the very inception of the universe. In
other words, our universe, and you and I, have incredibly small Kolmogorov
complexity.

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quietbritishjim
> everything has existed since the moment of the big bang

That's a very non-quantum view of things. For example, Hawking radiation [1]
relies on brand new matter-antimatter pairs of particles popping into
existence.

[1]
[https://en.wikipedia.org/wiki/Hawking_radiation](https://en.wikipedia.org/wiki/Hawking_radiation)

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yters
in that sense everything has very small kolmogorov sifficient statistic

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jaza
So the rate of star creation over time is b ^ x , where 0 < b <= 0.5 (i.e.
exponential decrease) ? Or perhaps e ^ (-bx) , where x >= 1 (i.e. exponential
decay) ? Sounds logical enough, considering that there was a lot more raw
material for star creation immediately after the big bang, than there has been
ever since then.

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graerg
This is colloquially referred to as the "Madau plot":
[https://ned.ipac.caltech.edu/level5/March14/Madau/Figures/fi...](https://ned.ipac.caltech.edu/level5/March14/Madau/Figures/fig9a.jpg)

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jiggawatts
I've always wondered if it's possible for there to be a steady-state universe,
where it expands continuously like it does now, but new matter is added at a
matching rate, keeping the density constant.

I suppose metallicity measurements of the oldest stars would exclude any such
scenario, but it's fun to think about anyway.

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mathieubordere
I'm not an astrophysicist, not even close (and I haven't read the article) but
making a statement like this seems bold. Who can predict how the universe will
evolve in the next 1000's of billions of years, there are just too many
unknowns, no?

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bjo590
We don't know with certainty how the universe will change in the future,
however we do have a decent understanding of how the universe works at a large
scale. We can use that understanding to make hypothesis that are backed by
both data and theory. This is one such hypothesis.

This is part of the scientific method. In astrophysics data is collected and
new experiments are done to improve theory, and it helps us better understand
the history of the universe and the future of the universe. Our understanding
will change over time.

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luxuryballs
I always wondered, if quantumness looks like observing something is what
solidifies it into being, then “almost all the stars” is really just reaching
a critical mass of observation? Somewhere beyond what we can see could be a
shit ton more stars in the making.

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cthalupa
Well, I think you're kind of conflating two things here.

It's not necessarily wrong to say that observing something solidifies it into
being, because it's not really a scientific term. There's too much
philosophical about the term 'being'

But the overall star formation of the universe isn't likely to be too tied to
having to observe or measure things at the quantum level. There's some caveats
there, and it's theoretically possible that a huge amount of mass could just
appear due to quantum tunneling, but isn't likely to occur in any place or
time relevant to us, or even our observable universe before heat death.

As for the second bit, the observable universe is the key, and we're back to
the philosophical. It's unlikely we'll be able to ever travel at the speed of
light, or faster than light. Even assuming we can get pretty close to the
speed of light, similar to what we see in relativistic jets, and could do so
today, and didn't care about making it back, we could really only reach about
3% of the stuff we can see now before the expansion of the universe takes it
forever out of our grasp. And the universe is expanding everywhere, so even if
there was other stuff that is, without expansion, moving "towards" us outside
of our view, it won't be able to overcome expansion.

So we'll never see it. Maybe somewhere out there in the cosmos is an area
where there's a ton of star formation left to go. But unless we can somehow
travel faster than light, it has zero ability to impact us. We won't ever be
able to observe it or confirm it. At that point, does it even actually exist,
as far as we are concerned? Does it make sense to even really think about it
or consider the possibility?

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vinniejames
Neat video on the projected future end of the universe
[https://youtu.be/uD4izuDMUQA](https://youtu.be/uD4izuDMUQA)

How close is it to this study?

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blickentwapft
Well that party ended a lot quicker than I expected.

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ggm
"everything in the universe exists, since the big bang"

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mensetmanusman
Infinity plus or minus

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m3kw9
And I say that should be enough.

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1024core
"640KB memory should be enough for everyone"

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mordymoop
The universe is believed to be infinite, so: infinity down, infinity to go!

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dylan604
Infinite space/size, not infinite mass though. The space between the mass is
expanding, but the mass is staying the same.

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867-5309
perhaps, until you factor in dark energy/matter

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saiya-jin
If mass of universe remains the same, so should any form of energy within it
(since they are interchangeable according to my layman's high school knowledge
of Einstein's stuff). Or you meant something else?

