- 3930 -
UNIVERSE - how big is it, really? That may be something that we actually will
never know. The size of the universe is
one of the fundamental questions of astrophysics. It also might be impossible
to answer.
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------------ 3930 - UNIVERSE - how big is it, really?
- Just look
out at the night sky; the glimmers you see are really pictures of the distant
past. That's because those stars, planets and galaxies are so far away that the
light from even the closest ones can take tens of thousands of years to reach
Earth.
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- The closer
an object is in the universe, the easier its distance is to measure. The
sun? The moon? All scientists have to do is shine a beam of
light upward and measure the amount of time it takes for that beam to bounce
off the moon's surface and back down to Earth.
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- But for the
most distant objects in our galaxy reaching them would take a very strong beam
of light. You would have to wait thousands of years to wait around for the beam
to bounce off the universe's distant exoplanets and return back to us?
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- Stars
change color as they age, and based on that color, scientists can estimate how
much energy, and light, those stars give off. Two stars that have the same
energy and brightness aren't going to appear the same from Earth if one of
those stars is much farther away. The farther one will naturally appear dimmer.
Scientists can compare a star's actual brightness with what we see from Earth
and use that difference to calculate how far away the star is.
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- What about
the absolute edge of the universe? How do scientists calculate distances to
objects that far away? The farther an
object is from Earth, the longer the light from that object takes to reach us.
Imagine that some of those objects are so far away that their light has taken
millions or even billions of years to reach us.
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- Now, imagine
that some objects' light takes so long to make that journey that in all the
billions of years of the universe, it still hasn't reached Earth. That's
exactly the problem that astronomers face.
We can only see a tiny, little “bubble” of the universe. And what's
outside of that? We don't really know.
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- By
calculating the size of that little bubble, scientists can estimate what's
outside of it. Scientists know that the
universe is 13.8 billion years old, give or take a few hundred million years.
That means that an object whose light has taken 13.8 billion years to reach us
should be the very farthest object we can see.
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- You might be
tempted to think that gives us an easy answer for the size of the universe:
13.8 billion light-years. But keep in mind that the universe is also
continuously expanding at an increasing rate. In the amount of time that light
has taken to reach us, the edge of the bubble has moved out.
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- Scientists
know just how far it's moved: 46.5 billion light-years away, based on
calculations of universe’s expansion since the big bang. Some scientists have used that number to try
and calculate what lies beyond the limit of what we can see.
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- Based on the
assumption that the universe has a curved shape, astronomers can look at the
patterns we see in the observable universe and use models to estimate how much
farther the rest of the universe extends. One study found that the actual
universe could be at least 250 times the size of the 46.5 billion light-years
we can actually see.
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- There's no
saying for sure whether the universe is finite or infinite. Because the
universe is expanding at an increasing rate, the outer edges of our observable
universe are actually moving outward faster than the speed of light. That means
that our universe's edges are moving away from us faster than their light can
reach us.
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- The universe
may be flat, but could still be shaped like a doughnut, weird patterns in
leftover light from the Big Bang suggest.
All observations so far suggest the universe is flat. In geometry,
"flatness" refers to the behavior of parallel lines as they go out to
infinity. Think of a tabletop: Lines that start out parallel will remain that
way as they extend along the table length.
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- In contrast,
look at Earth. Lines of longitude begin perfectly parallel to each other at the
equator but eventually converge at the
poles. The fact that parallel lines initially intersect reveals that Earth is
not flat.
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- The same
logic applies to the 3D universe. The cosmic microwave background (CMB) which
is light released when the cosmos was only 380,000 years old, now sits over 42
billion light-years away and features tiny fluctuations in temperature across
the sky.
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- Astronomers
have calculated the predicted size of those fluctuations compared with
observations. If their measured size differs from predictions, that means those
rays of light, which started out parallel, changed directions over space-time,
indicating that the geometry of the universe is curved.
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- But those
same measurements have revealed that, ignoring small-scale deflections from
galaxies and black holes, the overall geometry of the universe is “flat”. But, there's more than one kind of flat.
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- Draw
parallel lines on a piece of paper. Then wrap one end of the paper to connect
with the other, forming a cylinder. The lines remain parallel as they circle
the cylinder. In the language of mathematics, any cylinder is geometrically
flat but is said to have a different
”topology”. Close up both sides of the paper, and you make a “torus”, or
doughnut shape.
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- To get
another example of a weirdly flat shape, wrap a thin strip of paper in a
circle, but make a 180-degree twist in one end. The end result is a “Möbius
strip”, which is still geometrically flat, because parallel lines stay
parallel, even when they flip over each other.
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-
Mathematicians have discovered 18 possible “geometrically flat”, 3D
topologies. In each one, at least one dimension wraps up on itself, and
sometimes, they flip over like a Möbius strip or make partial rotations.
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- In such a
twisty universe, if we looked far away, we would see a (maybe upside-down) copy
of ourselves from a much younger age. If
the universe were 1 billion light-years across, astronomers would see a version
of the Milky Way galaxy as it was 1 billion years ago and, behind that, another
copy from 2 billion years ago, and so on.
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- If the
universe were a giant doughnut, astronomers could look in two directions to see
such copies. Astronomers have measured
the topology of the universe in multiple ways, from looking for duplicates of
patterns of galaxies to matching circles in the CMB. All evidence suggests the
universe is both “geometrically flat” and has a simple “unwrapped topology”.
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-
Observations have assumed that the universe wraps around itself in only
one dimension and does not have a more complicated topology. Also, observations
of the CMB have revealed some strange, unexplained anomalies, like large
patterns appearing where they shouldn't.
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- A universe
with a complicated topology could explain at least some of the anomalies in the
CMB. There may be a mirror image of us
somewhere in our twisty universe. Your
twisted comprehension of this situation may be comtirbuting to this confusion.
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March 25, 2023 UNIVERSE - how
big is it, really? 3930
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------ “Jim Detrick” -----------
---------------------
--- Sunday, March 26, 2023 ---------------------------
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