What lies beyond the observable universe?
13.8 billion years ago, the Big bang occured.
Our universe came into existence.
The universe was filled with matter, radiation, antimatter and existed in a hot, dense, expanding
state.
Even though it’s been billions of years since the Big Bang there is a cosmic limit
to how far we can see.
The universe has been expanding all this time, but that expansion rate is both finite and
well-measured by us.
If we were to calculate how far a photon emitted from the Big Bang could have travelled by
today, we come up with an upper limit to how far we can see in one direction: 46 Billion
light years.
That is what we call our observable universe.
But what lies beyond the observable universe?
What about the unobservable universe?
How can we even contemplate something that we cannot see?
Well in this video we will talk about all of that and more!!
But only if it were so easy to answer.
The universe is clumpy and cold today, and is also expanding and gravitating on the other
hand.
When we are looking at greater and greater distances, we don't only see far away in space,
we also look back in time, owing to the finite speed of light.
The more distant universe is more and more uniform, less clumpy and takes less time to
form larger, more complicated structures that require more time for gravity’s effects
to take place.
The early universe was a hot place.
It was so dense that even light couldn’t penetrate it, that's the reason why the universe
was opaque for about 300,000 years in the beginning.
But that changed slowly and the universe became transparent.
The expanding universe stretches the wavelength of all the light travelling through the universe.
As the wavelength stretches, it loses energy and becomes cooler.
This is why the colour of light also changes as it travels through the expanding universe,
which we call as redshift.
This observation concludes that the universe was hotter in the distant past.
We can measure the temperature of the universe as it is today, 13.8 Billion years after the
Big bang by simply looking at the leftover radiation from the dense and hot early state.
Today this radiation is commonly known as Cosmic microwave background radiation or CMB.
The cosmic microwave radiations prove the Big bang theory with extreme precision and
are an important evidence for the model.
They come with a blackbody spectrum and a temperature of 2.7K.
But an important point is that this cosmic microwave radiation evolves in energy with
the expansion of the universe.
A photon's energy is directly proportional to the inverse of its wavelength.
When the Universe was half its size, the photons from the Big Bang had double the energy, while
when the Universe was 10% of its current size, those photons had ten times the energy.
If we're willing to go back to when the Universe was just 0.092% its present size, we'll find
a Universe that's 1089 times hotter than it is today: around 3000 K.
This temperature is enough for the universe to ionize all the atoms in it.
As mentioned earlier, the entire early universe was hot, dense and in the form of an ionized
plasma.
Even light couldn't escape this early universe.
But you might be thinking how do we arrive at the size of the universe today?
Because the size of the universe is important if we are talking about seeing beyond the
observable universe.
We usually contemplate the size of the universe by understanding three things in tandem:
1.
How quickly the Universe is expanding today, something we can measure via a number of methods,
2.
How hot the Universe is today, which we know from looking at the radiation of the Cosmic
Microwave Background, 3. and what the Universe is made out of, including
matter, radiation, neutrinos, antimatter, dark matter, dark energy, and more.
With these three important things in mind, we can extrapolate the early stages of the
hot big bang and determine both the age and size of the universe.
We obtain our universe from the full range of available observations, including the cosmic
microwave background, but also data from supernovae, large-scale structural studies, and acoustic
baryonic oscillations.
13.8 billion years after the Big Bang, it now has a radius of 46.1 billion light years.
That is the limit of the observable.
Even further afield, and even something that has been moving at the speed of light since
the hot Big Bang will not have had enough time to reach us.
Over time, the age and size of the universe will increase, but there will always be a
limit to what we can observe.
So what can we say about that part of the universe that is outside the limits of our
observation?
We can only draw conclusions based on the laws of physics as we know them and the things
we can measure in our observable universe.
For example, we observe that the universe is spatially flat on the largest scales: it
is neither positively nor negatively curved, with an accuracy of 0.25%.
If we assume that our current laws of physics are correct, we can at least limit the size
of the universe before it curls back in on itself.
Observations from the Planck satellite and the Sloan Digital Sky study give us the best
data.
They tell us that when the universe curves towards itself and closes, the part that we
can see is indistinguishable from "not curved" in the sense that it is at least 250 times
the radius of the observable part.
This means that the unobservable universe, provided there is no topological insanity,
must be at least 23 trillion light years in diameter and contain a volume of space greater
than 15 million times the volume that we can observe.
However, if we are willing to speculate, we can argue quite strongly that the unobservable
universe should be significantly larger than that.
Based on the rate of expansion, the amount of dark energy we have, and the current cosmological
parameters of the universe, we can calculate what is known as the future limit of visibility
- the maximum distance we can observe.
Right now, in a universe 13.8 billion years old, our current visibility limit is 46 billion
light years.
Our future limit of visibility is about 33% higher: 61 billion light years.
There are galaxies at this time whose light is on its way to our eyes but has not yet
had a chance to reach us.
If we add up all the galaxies in the parts of the universe that we will one day see but
cannot access today, we would be surprised to learn that there are more undiscovered
galaxies than there are galaxies in the visible universe.
There are another 2.7 billion galaxies waiting to show us their light, in addition to the
2 billion we already have access to.
Compared to what the future holds, we currently only see 43% of the galaxies that we will
one day be able to observe.
Beyond our observable universe is the unobservable universe, which should look exactly like the
part we can see.
We know this through observations of the cosmic microwave background and the large-scale structure
of the universe.
The hot big bang may usher in the observable universe as we know it, but it does not mark
the birth of space and time themselves.
Before the Big Bang, the universe went through a period of cosmic inflation.
Instead of being filled with matter and radiation, and instead of being hot, the Universe was:
filled with energy inherent to space itself, expanding at a constant, exponential rate,
and creating new space so quickly that the smallest physical length scale, the Planck
length, would be stretched to the size of the presently observable Universe every 10-32
seconds.
It's true that in our region of the Universe, inflation came to an end.
But there are three questions we don't know the answer to that have a tremendous influence
on how big the Universe truly is, and whether it's infinite or not.
1.
How big was the region of the Universe, post-inflation, that created our hot Big Bang?
2.
Is the idea of "eternal inflation," where the Universe inflates eternally into the future
in at least some regions, correct?
3.
And, finally, how long did inflation go on prior to its end and the resultant hot Big
Bang?
It is possible that the universe in which inflation occurred was barely larger than
what we can observe.
It is possible that each year there is evidence of a "margin" to the location of inflation.
But it is also possible that the googol universe is times larger than what we can observe.
Until we can answer these questions, we may never know.
Aside from what we can see, we strongly suspect that there are many more universes out there,
like ours, with the same laws of physics, the same types of physical and cosmic structures,
and the same opportunities for complex life.
There should also be a finite size and scale for the "bubble" in which inflation ended,
and an exponentially large number of such bubbles contained in the largest spacetime
being inflated.
But as inconceivably large the entire universe, or multiverse, if you prefer, may not be infinite.
In fact, the universe should have a finite extent, unless inflation really lasted infinitely
or the universe was born infinitely large.
The biggest problem, however, is that we do not have enough information to definitively
answer the question.
We only know how to access the information that is available in our observable universe:
those 46 billion light years in all directions.
The answer to the greatest of all questions, whether the universe is finite or infinite,
may be encoded in the universe itself, but we cannot access it well enough to know it.
Until we figure it out or come up with a clever scheme to extend what physics is capable of,
we'll just have more and more possibilities.
So what do you think about the unobservable universe?
What lies beyond it?
The Multiverse or nothing?
How do you even define it?
Let us know what you think in the comment section below.
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