- 4433 - AGE OF THE UNIVERSE? - was it the beginning? If we count from the start of the hot Big Bang, we learn that the Universe is 13.8 billion years old, with only a very tiny (1%) degree of uncertainty. But what gives us the right to call the start of the hot Big Bang "the beginning," particularly if we now can confidently state that a period of cosmic inflation preceded it?
------------------------- 4433
- AGE OF THE UNIVERSE?
- was it the beginning?
-
- The reality is that we have to make choices,
and the start of the hot Big Bang is one of the earliest things we can be
certain about. Here's what the "age of the Universe" actually means.
-
- According to the theory of the hot Big Bang,
the Universe had a beginning. Originally known as “a day without a yesterday,”
this is one of the most controversial, philosophically mind-blowing pieces of
information we’ve come to accept as part of the scientific history of our
Universe.
-
- “How old is our Universe?”. The answer: 13.8 billion years. Why is this,
and when do we start counting? Why do
you start counting the age of the universe after 380,000 years have elapsed
after the Big Bang?
-
- The time “380,000 years after the Big Bang”
is of particular interest, but very few people mark that as the beginning of
the Universe; it is the beginning of something important, however.
-
- There are two different ways of measuring
the age of the Universe since the start of the hot Big Bang We can find “the oldest thing we know how to
measure its age” and conclude that the Universe must be at least that old. We can use what we know about the theory that
governs the Universe, general relativity, as well as our knowledge of what the
Universe is made out of plus how fast it’s expanding today to calculate how
long it’s been since the start of the hot Big Bang.
-
- The first method isn’t exactly a measurement
of how old the Universe is, but rather a sanity check: the Universe cannot be
younger than the things in it, so when we find things in it and measure their
ages, we conclude that the Universe must be at least that old.
-
- The life cycles of stars can be understood
in the context of the color/magnitude diagram. As the population of stars age,
they ‘turn off’, allowing us to date the age of the cluster in question. The
oldest globular star clusters have an age of over 13 billion years, but many
globulars also exhibit a second, more youthful population of stars alongside
the older one.
-
- Whenever and wherever stars are born, which
occurs whenever clouds of gas sufficiently collapse under their own gravity,
they come in a wide variety of sizes, colors, temperatures, and masses. The
largest, bluest, most massive stars contain the greatest amounts of nuclear
fuel, but perhaps paradoxically, those stars are actually the shortest lived.
-
- The reason is straightforward: in any
star’s core, where nuclear fusion occurs, it only occurs wherever temperatures
exceed 4 million Kelvin, and the higher the temperature, the greater the rate
of fusion.
-
- So the most massive stars might have the
most fuel available at the start, but that means they shine brightly as they
burn through their fuel quickly. In particular, the hottest regions in the core
will exhaust their fuel the fastest, leading the most massive stars to die the
most quickly. -
-
- The best method we have for measuring “How
old is a collection of stars?” is to examine globular clusters, which form
stars in isolation often all at once, and then never again. By looking at the
cooler, fainter stars that remain (and the lack of hotter, bluer, brighter,
more massive stars), we can state with confidence that the Universe must be at
least 12.5-13.0 billion years old.
-
- Measuring back in time and distance (to the
left of “today”) can inform how the Universe will evolve and
accelerate/decelerate far into the future. By linking the expansion rate to the
matter-and-energy contents of the Universe and measuring the expansion rate, we
can come up with an estimate for the amount of time that’s passed since the
start of the hot Big Bang.
-
- Similarly, we can take the known laws of
physics, like general relativity, and apply them to the expanding Universe.
That results in a set of equations — the Friedmann equations — that relate how
the Universe has expanded over its history to how fast it’s expanding today and
also the various forms of energy that are present inside of it.
-
- When we take the best suite of data that’s
available, including from the cosmic microwave background (CMB), which is made
of the light left over from the Big Bang, and from all the large-scale
clustering data we’ve collected, we get a straightforward answer that reveals
our cosmic history to us.
-
------------------------- We find that the Universe is made of:
-
------------------------- 68% dark energy,
------------------------- 27% dark matter,
------------------------- 4.9% normal matter,
------------------------- 0.1% neutrinos,
------------------------- 0.01% photons,
-
- And not an appreciable amount of anything
else. We also find that it’s expanding at a rate of 67 km/s/Mpc, which reveals
a Universe that’s 13.8 billion years old, if we extrapolate all the way back to
the instant of the Big Bang?
-
- Not entirely. There are three objections
you can make, each with varying degrees of validity. What about the Hubble
tension, or the fact that different measurement methods give a value for the
expansion rate that’s 74 km/s/Mpc, or 9% greater than the value quoted?
-
- If we measure an imprint from the early
Universe, like how far away different maximal “peaks” in density are from one
another in the expanding Universe, we get the earlier value of 67 km/s/Mpc with
the Universe’s constituents mentioned above.
-
- You might think that this would imply a
younger Universe, as “faster expansion” means it takes less time to trace the
Universe back to a condition where all the matter and energy was shrunk down to
a single point.
-
- But it turns out there are degeneracies
between various parameters in terms of “what makes up the Universe” and “how
fast is the Universe expanding,” meaning that if the expansion rate is 9%
greater, that forces us to slightly increase the amount of dark energy by a few
percent, at the expense of dark matter, which decreases by about the same
amount. The “age of the Universe” might shift a little bit, perhaps down to
13.6 billion years.
-
- As the Universe expands, it also cools,
enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and
finally galaxies to form. Without the Higgs giving mass to the particles in the
Universe at a very early, hot stage, none of this would have been possible.
-
- Should we start counting from 380,000
years, where the CMB we observe was emitted, or some other milestone, instead
of a nominal “t = 0” corresponding to the moment of the Big Bang? This is an interesting consideration, because
it makes sense only to extrapolate as far back as your data allows you to be
certain that the extrapolation is valid.
-
- We have two sets of signals that go back
farther: the abundance of the light elements created from Big Bang
nucleosynthesis, which takes place when only 3-4 minutes have elapsed since the
hot Big Bang, and the signals from the cosmic neutrino background that imprint
themselves into the CMB and the large-scale structure of the Universe, which
were created and frozen-in when only 1 second had elapsed since the hot Big
Bang.
-
- When we count back billions of years — 13.8 billion years — the uncertainty is in
the last digit: the “8” in 13.8 billion. If you’re off by 380,000 years, or a
few minutes or seconds for that matter, you won’t notice; that’s not
significant compared to the 13.8 billion figure.
-
- Okay, but the Universe didn’t really begin
with the hot Big Bang; cosmic inflation preceded it. So why not start at the
beginning of inflation?
-
- Our 92-billion-light-year diameter Universe,
today, was all contracted down into a single point. The start of the hot Big Bang corresponds to
a “singularity”, was once taken as a given from perhaps the 1920s, when the Big
Bang was first conceived, to the 1970s. But in the 1970s, we started to notice
some peculiar properties that didn’t seem to align with the notion of
extrapolating the hot Big Bang to those arbitrarily hot, dense, energetic, and
small states.
-
- If these three different regions of space
never had time to thermalize, share information, or transmit signals to one
another, then why are they all the same temperature? This is one of the
problems with the initial conditions of the Big Bang; how could these regions
all obtain the same temperature unless they started off that way, somehow?
-
- The Universe is spatially flat: where it
was as though the expansion rate and the total amount of matter-and-energy in
the Universe were perfectly balanced, down to the atom. That’s certainly
possible within the Big Bang paradigm, but is in no way predicted.
-
- We also saw that the Universe had the same
properties including temperatures and densities in regions that couldn’t have
communicated or exchanged information with one another since the start of the
hot Big Bang. And, for another, we didn’t see any leftover high-energy relics,
like the kinds we might expect if the Universe ever did reach those ultra-hot
states.
-
- One possibility that emerged was that the
Universe, before the hot Big Bang, was preceded by a period of exponential
expansion that set up and gave rise to the conditions we observe. The Universe
would be flat because inflation stretched it so it was indistinguishable from
flat, irrespective of what it was before. It would be the same temperature in
all directions because those now-disparate regions once overlapped, but
inflation drove them apart. And there would be no high-energy relics because
the Universe never achieved those arbitrarily high temperatures, but only
reheated, after the end of inflation, to a finite temperature that was below
the Planck scale.
-
- What set inflation apart from other
speculations, however, was its ability to make predictions that differed from
the hot Big Bang’s if there were no inflation. Many of these predictions have
been borne out by later observations:
-
----------------- the prediction of an almost scale-invariant
spectrum of density fluctuations, with a slight tilt to it,
----------------- where all of the fluctuations would be
adiabatic, and not isocurvature, in nature,
including the existence of
fluctuations on scales larger than the cosmic horizon set by the speed of
light,
----------------- where the Universe reached a maximum
temperature, as indicated by the CMB, that was well below the Planck scale.
- All of these predictions have been
subsequently confirmed, implying that there was a period of exponential
expansion prior to the start of the hot Big Bang. But how long did that period last, and what
came before it?
-
- For the first question of how long it
lasted, that’s a question where we only have a lower limit, but there is no
upper limit set by data. Inflation must have resulted in the Universe
“doubling” in size at least a few hundred times, but if each “doubling” only
takes something like 10^-35 seconds, then that only tells us that the Universe
must have undergone inflation for at least ~10^-32 seconds. It could have
lasted nanoseconds, seconds, years, trillions of years, googols of years, or
even longer before ending and giving rise to the hot Big Bang.
-
- In an inflationary scenario, we never reach
a singularity, where space goes to a singular state; instead, it can only get
arbitrarily small in the past, while time continues to go backward forever.
Only the last minuscule fraction of a second, from the end of inflation,
imprints itself on our observable Universe today. The size of the
now-observable Universe could’ve been no smaller than about 1 cubic meter in
volume at the start of the hot Big Bang.
-
- But the answer is also, “It probably didn’t
go on for an infinite amount of time,” when it comes to inflation. Although
there may be loopholes that allow us to avoid an initial singularity, there are
some very compelling theorems that strongly suggest that inflation arose from a
pre-inflationary state that may have been singular. It’s unknown what the
physical mechanism was that started it off, or if our presently understood laws
of physics even apply to those early times.
-
- But one thing is for certain: when we talk
about the “age of the Universe,” we’re talking about the “age of the Universe
that we can observe,” which includes the Universe going back to the start of
the hot Big Bang and the tiny fraction-of-a-second over which the final moments
of inflation left an imprint on our Universe.
-
- There was almost certainly more inflation
before the final piece of it that left observable signals for us to see, and
there was almost certainly something else before the onset of inflation, but
how long they lasted, what they were like, and what caused them to begin are
not questions that science has answered. The Universe we observe is 13.8
billion years old, but what came before it (and for how long) is still firmly
in the realm of speculation.
-
-
April 14, 2023 AGE
OF THE UNIVERSE?
- was it the beginning? 4433
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--------------------- --- Tuesday, April 16, 2024
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