- 4343 - DARK ENERGY - expanding the Universe? It’s only because we came along when we did, at this golden cosmic time, that we can perceive what the Universe is actually made of. Dark energy is real, began dominating our Universe at an age of 7.8 billion years old, and determines our cosmic fate from here on out!
--------------- 4343 - DARK ENERGY - expanding the Universe?
- In the beginning the Universe behaved as
though it were made solely of matter and radiation, with no effects from dark
energy appearing from the first fractions-of-a-second to the first several
billion years.
-
- As the Universe expanded and cooled, the
densities of matter and radiation began to plummet, eventually dropping so low
that a new form of energy, “dark energy”, became important to the Universe's
evolution. As dark energy rises to prominence, the expanding Universe's
behavior changes dramatically, and structure on the largest scales stops
gravitationally growing and instead gets torn apart.
-
- ERxamining the galaxies, quasars, and other
forms of matter that appear billions of light-years away, is seeing those
objects not as they are today, but as they were in the distant past: back when
their light was first emitted.
-
- At those earlier times, the Universe was
hotter, denser, and filled with smaller, younger, less-evolved galaxies. The
light we see from way back in our Universe’s history only arrives at our eyes
after journeying across these vast cosmic distances, and only after that light
has been stretched by the expanding fabric of space.
-
- It’s these early signals, and the process
of how that light gets stretched to longer wavelengths, redshifted, more
severely as we look to more and more distant objects, that teach us how the
Universe has expanded throughout its history.
-
- We learned that the Universe wasn’t just
expanding, but that distant objects appear to speed up, faster and faster, as
they mutually recede from one another.
This is the discovery of the “accelerated expansion” of the Universe.
That’s how we discovered dark energy and measured its properties, changing our
conception of the Universe forever.
-
- At the start of the hot Big Bang, the
Universe was rapidly expanding and filled with high-energy, very densely
packed, ultra-relativistic quanta. An early stage of radiation domination gave
way to several later stages where radiation was sub-dominant, but never went
away completely, while matter then clumped into gas clouds, stars, star
clusters, galaxies, and even richer structures over time, all while the
Universe continues expanding.
-
- In the beginning no light signals would be
able to freely propagate from one region to the other; you’d have to wait until
the Universe reached an age of 380,000 years old, as that was when the Universe
became transparent to light. The cosmic microwave background, initially
visible, would fade to infrared and then microwave wavelengths.
-
- As time continued to pass, you’d see
molecular clouds form and contract, followed by stars and black holes forming
in a slew of early nebulae, followed by the mergers of star clusters, leading
to proto-galaxies. Those proto-galaxies would then merge, gravitate, and grow,
evolving into full-fledged galaxies that came to exist within groups and
clusters. Eventually, those early galaxies would evolve into the more familiar
modern-day galaxies, going through long, quiet eras punctuated by bursts of
star-formation.
-
- Galaxies comparable to the present-day
Milky Way are numerous, but younger galaxies that are Milky Way-like are
inherently smaller, bluer, more chaotic, and richer in gas in general than the
galaxies we see today. For the first galaxies of all, this effect goes to the
extreme. As far back as we’ve ever seen, galaxies obey these rules.
-
- One of the great properties of the Universe
is that the laws of physics appear to be constant: unchanging throughout time.
This means that atoms always have and always will absorb and emit light at very
specific wavelengths: wavelengths that are the same everywhere, and determined
by the energy levels that the electrons within the atom occupy.
-
- By identifying series of atomic absorption
or emission lines that correspond to the same element, we can then measure how
our observations are shifted in wavelength relative to the wavelengths we’d
observe if that same atom were at rest in a laboratory.
-
- The shift, known as a “redshift” if it
gets stretched to longer wavelengths or a blueshift if it gets compressed to
shorter wavelengths, is almost always observed to be a redshift for distant
galaxies, with redshift generally increasing with distance. By measuring both
of those properties, redshift and distance together, across a wide enough array
of objects with different properties, we can use those measurements to
reconstruct the history of the expanding Universe.
-
- The supernova data ever since 1998, has
pointed toward a Universe that expands in a particular fashion that requires
something beyond matter, radiation, and spatial curvature, a new form of energy
that drives the expansion, known as dark energy. The supernovae all fall along
the line that our standard cosmological model predicts, with even the
highest-redshift, most far-flung “Type Ia supernovae” adhering to this simple
relation.
-
- Measuring the distance and redshift of a
wide variety of objects across the Universe allows us to infer the answer to
our initial hypothetical scenario of how two points in space, one corresponding
to ourselves and one corresponding to a distant, unbound galaxy, recede from
one another across cosmic time.
-
- When we observe the light from a relatively
nearby galaxy, its light is redshifted by an amount that corresponds to the
amount the Universe has expanded from when that light was emitted until we
observe it. The light from a slightly more distant galaxy is redshifted by a
slightly greater amount: by the same amount as the nearer galaxy plus the
additional amount of expansion that occurred due to its greater distance.
-
- As we accumulate more objects at greater
and greater distances, we can use that data to construct a curve that teaches
us how the Universe has expanded over its cosmic history. The way the Universe expands over its cosmic
history is related to and determined by the different forms of energy that are
present throughout the Universe over its history.
-
- A plot of the apparent expansion rate
(y-axis) vs. distance (x-axis) is consistent with a Universe that expanded
faster in the past, but where distant galaxies are accelerating in their
recession today.
-
- Once we know what the Universe is made of,
we can use the information we have about how various forms of energy evolve
with time to answer that initial question: what we would see if we could track
a single, individual galaxy’s distance and redshift (as seen from our
perspective) throughout the history of the Universe.
-
- In the earliest stages, the light that
first arrived from a distant object would tell you its distance and its
redshift. Compared to what we see today, the distance to that object would have
been relatively small, while the redshift that would have been observed would
have been quite large compared to what we see today. -
- That redshift, if we interpret it as a
“Doppler shift,” can be made to correspond to an apparent recession speed: how
quickly the object in question appears to be moving away from us.
-
- In reality, it isn’t that the object’s
motion is causing the redshift, although motion toward (blueshift) or away from
(redshift) an observer can certainly cause that effect. Instead, it’s the fact
that the light is traveling through the fabric of space and that the fabric expands while the light
travels that causes the redshift we observe.
-
- Initially, the distances to other
extragalactic objects would be small while the redshifts would be large: we
would infer that this distant galaxy is speeding away from us at a very rapid
rate. But as we allow the clock to run forward, both the distances to and the
inferred velocity of those objects would change by significant amounts, but in
opposite directions from one another.
-
- The distances get larger and larger over
time, as the Universe continues to expand. This pushes all objects that aren’t
gravitationally bound mutually away from one another, increasing the measured
distance between them. From the perspective of any one galaxy, all distant,
unbound galaxies continue to move farther and farther away as time goes on.
-
- The Universe’s expansion rate changes, and
it changes dependent on the total matter and energy density present in the
Universe. Since an increasing volume means a decreasing energy density, the
expansion rate drops, and the distant galaxy appears to move away from us at a
slower and slower speed, as though the initial expansion drove things apart,
and gravity attempts, however unsuccessfully, to pull them back together.
-
- Light may be emitted at a particular
wavelength, but the expansion of the Universe will stretch it as it travels.
Light emitted in the ultraviolet will be shifted all the way into the infrared
when considering a galaxy whose light arrives from 13 billion years ago. The
more the expansion of the Universe accelerates, the greater the light from
distant objects will be redshifted and the fainter it will appear.
-
- This makes sense when you think about the
expanding Universe in the context of the Big Bang. There is a great cosmic race
going on: between gravity, working to pull everything back together, and the
initial expansion rate, working to drive everything apart.
-
- The race has been underway for
13,800,000,000 years, and the Big Bang
was the starting gun. Everything in the Universe begins moving away from
everything else initially, at an extremely rapid rate to start, while gravity
works as hard as it can to pull everything back together.
-
- If there were too much matter in the
Universe, everything would expand only until a point, as the Universe reached a
maximum size and then the expansion reversed, and things began contracting.
Eventually, the Universe would recollapse, and everything would end in a “Big
Crunch”.
-
- On the other hand, if there were too little
matter, the expansion would continue forever, with the expansion rate
decreasing but never stopping or reversing, while the apparent recession
velocities would forever decrease but never reach zero.
-
- Or you could imagine what we call a
“Goldilocks” case: where the Universe lived on the border between those two
prior scenarios. The Universe would expand forever, but the expansion rate
would approach zero. If there were one more atom in the Universe, it would
recollapse, but without that atom, things just keep expanding, albeit as slowly
as the laws of physics allow.
-
- The Big Bang, on its own, offers no
explanation as to why the initial expansion rate at the moment of the
Universe’s birth balances the total energy density so perfectly, leaving no
room for spatial curvature at all and a perfectly flat Universe. Our Universe
appears perfectly “spatially flat”, with the initial total energy density and
the initial expansion rate balancing one another to at least some 20+
significant digits.
-
- That last case was consistent with what
we’d see happening for a long time: for billions of years, in the case of our
Universe. An individual galaxy appears to move away from us at an incredibly
fast rate, but then its recession velocity drops as the matter and radiation
densities drop. Since it’s the total energy density at any particular instant
that determines the Universe’s expansion rate, and the expansion rate in turn
determines what we infer the recession speed of a galaxy to be, this all makes intuitive
sense.
-
- But,
after a few billion years, something fishy begins to occur. Instead of
approaching zero, the expansion rate starts to decrease at a slower rate than
one would expect, and a distant galaxy’s recession speed doesn’t drop in the
same fashion anymore. Once the Universe reaches an age that’s 7.8 billion years
after the Big Bang, things start to get weird: these distant galaxies stop
slowing down in their recession entirely, and appear to “coast” in the sense
that they move away from us at a constant speed from moment-to-moment, as
though the expansion had stopped decelerating.
-
- And then, as the Universe continues to age,
the recession speeds no longer remain constant, nor do they go back to
decreasing. Instead, these distant galaxies appear to recede from us (and one
another) more and more quickly. It’s as though some effect is causing the
expansion to neither decelerate nor remain constant, but to actually increase
and accelerate!
-
- The expected fates of the Universe all
correspond to a Universe where matter and energy fight against the initial
expansion rate. In our observed Universe, a cosmic acceleration is caused by
some type of dark energy. All of these Universes are governed by the Friedmann
equations, which relate the expansion of the Universe to the various types of
matter and energy present within it. The Universe with dark energy, the expansion rate makes a hard transition
from decelerating to accelerating about 6 billion years ago.
-
- The Universe cannot simply be filled with
merely matter and radiation. Even adding in neutrinos, black holes, dark matter
and more won’t allow us to successfully account for everything. In addition to
all of those entities, we also require something novel, known as dark energy: a
form of energy inherent to space itself.
-
- As the Universe expands, dark energy
doesn’t dilute, but rather remains at a constant density. Everything else,
including all forms of matter and radiation, dilute as the Universe expands, as
the number of particles remains fixed but the volume they occupy increases: a
consequence of the expanding Universe. Only dark energy, inherent to space
itself, remains at a constant energy density.
-
- After 7.8 billion years, the matter density
drops far enough that the effects of dark energy begin to become important. 7.8
billion years after the Big Bang, when the dark energy density has grown to be
as large as half the matter density, it reaches the critical value to cause a
distant galaxy to stop decelerating from our perspective, and begin
accelerating instead. The repulsive effects of dark energy on the Universe’s
expansion exactly counteract the attractive effects of matter.
-
- But time doesn’t stop here. Instead, it
continues forward, and the matter density continues to drop. Once 7.8 billion
years on the cosmic clock ticks by, dark energy now becomes more important than
matter and radiation as far as the expansion rate is concerned. Distant
galaxies reached their minimum recession speed at that time, but then will
appear to speed up once again.
-
- In a Universe that comes to be dominated by
dark energy, there are four regions: one where everything within it is
reachable and observable, one where everything is observable but unreachable,
one where things will someday be observable, and one where things will never be
observable.
-
- As time marches forward, distant objects
not bound to one another will recede from each other’s perspective at a faster
and faster rate. By the time the Universe is 9.2 billion years old, right when
our Solar System is forming, the matter density will have dropped below the
dark energy density.
-
- By the present day, 13.8 billion years
after the Big Bang, dark energy accounts for approximately 70% of the total
energy in the Universe; when the Universe reaches twice its present age, dark
energy will account for over 95% of the Universe’s total energy. Throughout all
that time, distant galaxies will continue to speed up, faster and faster, in
their apparent recession from our perspective.
-
- For the past 6 billion years, the
Universe’s expansion has been accelerating, meaning that any distant galaxy we
monitor appears to recede from us at an ever-increasing speed. Any galaxy
currently at a distance of approximately 18 billion light-years from us now
appears to recede away faster than the speed of light, meaning there’s nothing
we can ever do to reach or contact it again.
-
- Given that the Universe is already 46
billion light-years in radius, this means that 94% of the galaxies in the
Universe are already forever beyond our reach.
-
- For billions of years, dark energy’s
density would have been tiny compared to the density of matter, meaning its
effects would have been undetectable if humans had arisen too early. Tens of
billions of years from now, dark energy will have pushed everything beyond our
Local Group far away from us; the merged remains of the Local Group will be the
only galaxy remaining.
-
- It’s only because we came along when we
did, at this golden cosmic time, that we can perceive what the Universe is
actually made of. Dark energy is real, began dominating our Universe at an age
of 7.8 billion years old, and determines our cosmic fate from here on out!
-
-
February 8, 2023 DARK
ENERGY - expanding the Universe? 4343
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------ “Jim Detrick” -----------
--------------------- --- Friday, February 9,
2024
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