Earth’s surface is close to 200 million square miles in total, with a circumference of over
24,000 miles at the equator and a population nearing eight billion. As big as this might
sound to humans, however, it’s absolutely nothing in the grand scheme of things. The
universe is incomprehensibly vast, and there’s no clear consensus on exactly how far it stretches.
This is Unveiled, and today we’re answering the extraordinary question;
how big is the universe really?
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Before getting into the size of the universe, it’s useful to first think about the age of the
universe. Surprisingly, we didn’t have a really accurate estimate for the universe’s age until
the 2010s. Thanks to data gathered by the Planck space observatory, we know today that the universe
is roughly 13.8 billion years old, give or take about 20 million years. We got this number by
studying the universe’s rate of expansion and the cosmic microwave background radiation (or CMB),
the residual radiation left by the Big Bang. The CMB dates to around 380,000 years after the Big
Bang, just as particles began forming atoms. But you’d be wrong in assuming that, because the
universe is 13.8 billion years old, the observable universe can only reach 13.8 billion lightyears in
any direction - which would give the universe a total diameter of almost 28 billion lightyears.
In fact, scientists have found that the observable universe is a lot larger than that,
with a diameter of around 93 billion lightyears. This means that we have an observable universe
that is far larger than seems possible – but there are a few reasons why this is the case.
The biggest reason is cosmic expansion, as the space between spaces grows at an increasing rate.
There is, however, a contentious theory among a small group of scientists that
contrary to what Einstein’s theories outline, the speed of light is not a constant.
These scientists have been arguing since the 1990s that the speed of light was
once significantly faster and is, in fact, slowing down. And it is true that at 186,000
miles per second, the speed of light is pretty slow relative to the size of the universe.
That’s still a niche theory, however, and the most widely accepted explanation for this phenomenon
is still cosmic expansion. The universe sprang from the initial singularity at the dawn of time,
the point of infinite heat, matter, and density from which everything in existence came.
Since the Big Bang, the universe has been expanding. However, for a long time, we believed
that the rate of expansion was slowing down. This isn’t actually the case at all; in 1998, studies
that measured the distance to two large supernovae found, shockingly, that cosmic expansion is
accelerating. Those supernovae were, and still are, moving away from us and from each other,
increasingly quickly. The way we know things are moving away from us here is redshift. Essentially,
because red wavelengths of light are longer with a lower frequency than blue wavelengths of light,
the longer wavelengths are the ones able to reach us as an object moves further away.
This means objects visually appear redder and redder. Redshift is invaluable in calculating
distances and speeds. And it’s important to note we do have some estimates of how fast cosmic
expansion is happening, too – and interestingly, it’s probably happening faster than the speed of
light. One estimate puts the speed of inflation at “68 kilometers a second per megaparsec”,
with a megaparsec being over 3 million lightyears. That makes expansion a LOT faster than the speed
of light. But it doesn’t break the idea that nothing can travel faster than the speed of light,
because it doesn’t refer to an object moving through space.
The expansion of space is the reason the observable universe has grown to be 93 billion
lightyears from one “edge” to the other. It’s important to note that “observable” extends beyond
the “visible universe”, which is what’s currently visible based on our eyes and instruments.
There are things way beyond the range of our perception that still constitute the
observable universe because they’re the areas that the light from the Big Bang has reached.
However, while we do know, more or less, how big the observable universe is, the unobservable
universe is an entirely different story! In truth, we just don’t know how big the
unobservable universe is, but we can make some solid guesses based on what shape the universe
might be. Specifically, though we can’t know for sure, the most likely possibility is that
the universe is mostly flat, but slightly curved. The degree the universe is curved is minuscule,
but it does mean there’s a limit to how big the universe can be until it literally comes full
circle. If the laws of physics beyond the boundaries of the observable universe are
the same as the laws within it (and we have no real reason to believe they wouldn’t be)
then that would make the universe around 23 trillion lightyears across. If the universe
doesn’t have this degree of curvature, then it could potentially be an infinite flat plane,
or it could be a closed sphere the size of which we have no frame of reference to estimate.
But it is true that many complex phenomena in the universe are flat; the solar system, for example,
is a mostly flat plane, as are galaxies. So, it’s not difficult to imagine that the universe
itself also occupies a flat shape in a way. In a lot of ways, humanity is still at the very
beginning of our cosmological journey. The Planck spacecraft responsible for many of the findings
any calculation of universal scale relies on did only start working in 2009, shockingly recently,
and we’ve come leaps and bounds because of it in our understanding of the universe.
Only a hundred years ago did we work out how to calculate the distances between stars by examining
how bright they were, a technique pioneered by American astronomer Henrietta Leavitt.
Her calculations gave us the “standard candle” model, where we can extrapolate distances by
looking at light sources. Often, the light sources used are incredibly bright supernovae.
This is still the cornerstone of our methods to work out the vast distances of outer space,
and we had no idea about it until the early 1900s. To come so far since then, to construct
a spacecraft as advanced as Planck and be able to image and observe the cosmic microwave background
radiation of the entire universe, is an astonishing feat! Now we know there are
an estimated two hundred billion of stars in the Milky Way, and we know how far away we are from
the galactic center. We even know that our closest neighboring galaxy, Andromeda, is on a collision
course for us. None of these calculations were possible just a few decades ago.
Ultimately, we may never know the true size of the universe because it’s always growing
at an alarming rate. It’s not yet clear what the endpoint of cosmic inflation will be,
as the universe progressively becomes almost entirely made of dark energy while its entropy
increases. Cosmic inflation might one day slow down, but it probably won’t, leaving Earth even
more lonely and isolated than it already is. When cosmic inflation reaches a certain point, the
universe might finally die in “heat death”, where no new energy or heat can ever be created and
everything settles to the exact same temperature and becomes uniform dark energy. Alternately, the
universe could experience “cold death” if it keeps expanding forever, where everything becomes much
too cold and far apart to ever support life. Ultimately, we just don’t know how large the
total universe is – it could be anywhere from 23 trillion lightyears until it circles back
on itself, to an infinite plane of existence. But we’ve still got our hands full with the
parts of the universe we can examine for the time being. And that’s how big the universe really is.
What do you think? Is there anything we missed? Let us know in the comments, check out these other
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