The universe is precisely 13.8 billion year old - or so our best scientific methods tell
us. But how do you learn the age of the universe when there’s no trace left of its beginnings?
We see the same constellations on the night sky as did our astronomer ancestors. Familiar
star-maps are recorded in cave paintings tens of thousands of years old. We could be forgiven
for assuming that the universe above is fixed and unchanging. But the Earth was also thought
to be timeless - until we learned to see billions
of years of change in its geological layers. Well, the universe also changes and
also had a beginning. In our recent episode we learned how we calculate
the age of the Earth based on radioactive decay in its most ancient rocks.
But there are no rocks from the beginning of the universe. There aren’t even any
photons from the time right after the Big Bang. So today we go deeper into
deep time to understand how we can possibly have learned the birthday of
the universe. Actually, you know what? Forget for a moment
knowing the age of the universe - our knowledge that the universe
even had a beginning is relatively recent. Let’s talk about that
first. The discovery of the beginning of the universe corresponds to our discovery
that there even is a universe outside the Milky Way galaxy. This is something
we explored in our STELLAR series, but it’s worth a review.
At the beginning of the 20th century, astronomers were arguing about the
nature of these faint, fuzzy patches of light on the sky known as spiral
nebulae. Were they blobs of gas in the Milky Way, or vast, distant groups of
stars - other “Milky Ways”, or as Immanuel Kant called them, island
universes. We now call them galaxies. It all changed in the 1920s. First
Vesto Slipher found that the spiral nebulae are moving away from us at
incredible speeds, based on their Doppler shift - the lengthening of the
wavelengths of their light due to their motion. Then Edwin Hubble figured out the distances
to these objects by watching their stars pulse. He located a star type
called a Cepheid variable, whose pulsation rate is proportional to its brightness
- encapsulated in a simple period-luminosity relation. By measuring the
pulsation rate he could calculate its true brightness, undimmed by
distance. He figured out that in order for these stars to appear so faint in
his telescope, they had to be many millions of light years away.
Combining Slipher’s velocities or redshifts and Hubble’s distances, we
learned that essentially galaxies are racing away from us. And the further
the galaxy the faster it seemed to be retreating.
This was a shocking discovery. Until then, science had provided no reason
to imagine that the universe was anything but static – that it had always
been there, and that it had always looked pretty much the way it looks now.
The new picture was far more dynamic. We live in an evolving, expanding
universe. The recession of the galaxies makes perfect
sense in the context of Einstein’s then-new general theory of relativity.
The Russian cosmologist Alexander Friedmann solved Einstein’s equations
and found the possibility of a universe that could change in size - a
result that Einstein himself dismissed at the time. In fact even before
Hubble’s observation the Belgian physicist and Jesuit priest, Georges Lemaître,
suggested that Vesto Slipher’s observed redshifts could be a sign of the
universe’s expansion. Lemaitre put forward the idea that the world
began in a state that he referred to as a “primeval atom.” Think of it this
way - if the universe is expanding now, then in the past it was smaller. Rewind
the clock according to the raw Friedman equations and there’s no alternative
- the universe must have once been in an extremely hot, dense state, and
expanded from there. Astronomers call this the big bang model of
the universe. Now the name “big bang” was coined by the astronomer Fred
Hoyle during a 1950 BBC radio broadcast. But not to popularize the idea
- rather to mock it. Hoyle was the last great holdout against the idea of a dynamical
universe with a finite age sticking with his own “steady state” model.
This idea that the universe has a beginning brings to mind the creation
stories of various mythic or religious traditions. Some have even tried to
claim that the Big Bang is a validation of their tradition. Pope Pius XII
certainly thought so. But priest or no, Georges Lemaître himself disagreed,
stating: “As far as I can see, such a theory remains entirely outside any
metaphysical or religious question.” So if we can predict that the big bang by rewinding
the Friedman equations, surely we can also predict how long ago it
happened. Think of it this way - if a particular galaxy is racing away from
us, and we know how fast it’s moving and how far away it is now, we can figure
out how long it must have taken to cover that distance. In other words,
how long ago was it right on top of us? Even doing that for a single
galaxy gets us a rough estimate of when the Big Bang happened. However
to accurately calculate the age of the universe this way you need
to account for a few more things. First up there’s the fact that galaxies
aren’t only moving away from us due to the expansion of the universe - they also
have random motion as they’re
tugged by the gravitational fields of nearby galaxies and clusters. We can
deal with these “peculiar velocities” just by averaging over many galaxies.
Then there’s the fact that the expansion rate has changed over time. You
can’t just assume that the galaxies were always moving away at their current
speeds. Now we’ll come back to that later. For now, we’ll see that we can
do a lot just having a good idea of the current expansion rate.
That rate is encapsulated in something we call the Hubble constant, which
just tells how fast a galaxy is moving away from us given its distance from us - the
more distant, the faster it’s moving away. In the near-century since Edwin
Hubble’s great discovery, thousands of astronomers have devoted their lives
to measuring the Hubble constant - and a big part of the motivation is that it
tells us the age of the universe. In fact if you take the fraction one over the
Hubble constant - in the right units - you have the age of the universe, at
least assuming no gravity. And if your Hubble constant was determined by
looking at lots of galaxies, then it gives you an age averaged over lots of
galaxies - and that averages out those pesky peculiar velocities.
And when astronomers did this calculation in the early 1930s, they figured
that the universe is a bit less than two billion years old. Even then that figure
didn’t sound right. By then, geologists had already found rocks here on
earth that were at least 3 billion years old! So there was a problem - but it wasn’t a
problem with the theory, it was a problem with the observations. Edwin Hubble
had got the distances to the galaxies wrong. It turns out that Cepheid
variable stars come in two types, with two different period-luminosity relations.
Hubble had observed a brighter variety of Cepheids in distant galaxies,
but he then used a period- luminosity relationship measured from a different,
fainter class of Cepheids that were measured in the Milky Way. So, he
measured the periods of his Cepheids and calculated what he thought were
their true luminosities - but the numbers he got were too low. He thought
they were intrinsically fainter than they really were, and so concluded that
they had to be closer to us than they really were. In fact he got distances
wrong about a factor of two too small. Overnight the universe doubled in size.
This whole 2-different-types of Cepheid variable issue was figured out by
Walter Baade. In 1943, beneath the dark skies of wartime blackouts, he used
the 200-inch telescope at Mt. Palomar to peer deeper than anyone had
before into the Andromeda galaxy. With a newly calibrated Cepheid period-
luminosity relation, and also incorporating other distance measures which
I’ll come back to, Baade calculated a new age for the universe. At a
conference in 1952, he announced that it must be 3.6 billion years old. At
last, the universe was older than the earth – although still a way off our
modern value.
To understand the next step in improving this number, I need to explain
another method astronomers had been using to get distances to galaxies, and
so calculate the Hubble constant and the age of the universe. It’s similar to
the Cepheid method because it’s based on comparing the expected, true
brightnesses of stars with the apparent, distance-dimmed brightness. It seems
fair to assume that on average the brightest stars in any galaxy have pretty
much the same average brightness from one galaxy to another. So if the
bright stars in a given galaxy appear faint on average in our telescopes, it
means the galaxy is further away, if they appear brighter, then it’s closer.
But astronomers had originally made an awkward mistake - they had been
counting bright clouds of hydrogen gas - so-called HII regions - as stars,
which threw their numbers off. It was a young astronomer named Alan
Sandage who figured this out. Sandage was a student of Walter Baade and
also worked closely with Edwin Hubble. By eliminating the problem with HII Regions
he came up with a new age estimate for the universe - 5.5 billion
years. That was in the 50s. As the decade went on,
Sandage continued to revise those estimates. He eventually concluded that
the expansion rate of the universe – the Hubble constant – was about
75 kilometers per second per megaparsec. Now, don't worry about those units too much,
but it gives an age of the universe of around 13 billion years, assuming no gravity and no dark energy. But that’s
remarkably close to the currently accepted value. Sandage figured out
that with gravity playing its part to slow down the expansion, the universe must
have been expanding faster in the past and so it should have taken less
than 13 billion years to reach its current size. He estimated its age as between
7 and 13 billion years old. Now there were some moments of confusion. For a while
we thought we’d found globular clusters - ancient, dense groups of stars
- that were 15 billion years old. As bad as finding rocks older than the universe.
But as our understanding of stellar evolution improved, those ages came
down to under 13 billion years. Now to arrive at our current, very precise value
of 13.8 billion years, you need to take into account the effect of matter slowing
down expansion through its gravity and also the effect of dark energy speeding
it up. Those can be put into the Friedman equations and a bit of calculus
later gets you an age. The real hard part is figuring out how much of all
that stuff there actually is. The mass of the universe - which is mostly in
dark matter - can be found by adding up the gravitational effect in galaxies
and in galaxy clusters, and also by tracking the past expansion history of
the universe to observe the slowing effect of all of those galaxies on the whole
universe. It was in an attempt to do the latter that astronomers discovered
the anti-gravitational effect of dark energy, and had to start adding that into
their equations also. But that’s a story for another playlist.
Long story short, the accelerating effect of Dark Energy counters the slowing effect of gravity
to ultimately give us an age of the universe very close to the roughly 13.8 billion years that we get.
from assuming the expansion rate hasn't changed at all.
These days, the gold standard for measuring the age of the universe is to get
the matter content, and the dark energy content, and the expansion rate, and more all from
the one source - the cosmic microwave background radiation - the oldest
light we can see, released when the universe was a mere 400,000 years old
and MUCH smaller and hotter than it is today. And for the details of the cosmic
microwave background and how we use it to calculate the contents of the
universe? Guess what - here are some episodes we prepared earlier.
Based on analysis of the cosmic microwave background map produced by
the Planck satellite we get the relative amounts of matter and dark energy
and a number of other quantities. Quantities important for plugging into the
Friedman equations, which define the way the universe would expand in the
many billions of years following the release of that radiation
and also how it had expanded before that time. There are other
approaches to getting the necessary numbers to fuel the Friedman equations
- and despite some intriguing conflicts - different methods are mostly in
agreement. They converge on a single number. It’s been 13.8 billion years
since the fiery beginning of time-as-we-know it, the birthday of spacetime.
As you can well imagine in these difficult times, every little bit of help is huge. There's no way we could be
so consistent in our releases if wasn't for your support. And that's especially true for
our Patreaon supporters. Your contributions, large and small, are a big part of what makes
this show possible. Today I'd like to give an extra huge thank you to Radu Negulescu
, who's supporting us at the big bang level. Radu as an official representative of both space
and time, I want to thank you on behalf of the universe for ensuring that we keep paying attention to it
despite everything.
So, last time we talked wormholes - one of the most awesome entities in general relativity
that probably don't exist.
Devansh Rana asks Can wormholes exist without black holes ie without the event horizon.
Well the Schwarzschild wormhole definitely has an event horizon - although at its most open, entering
the event horizon ejects you into the parallel universe. You can construct a wormhole in
general relativity that does not have an event horizon, but you need the non-existent negative energy to do
so and keep it open long enough to traverse. The reality is that, in general, traversible
wormholes probably can't exist - and having an event horizon is one way for the universe
to enforce that.
Magnum Polmatier summarized that better with "sort of/not really"
Lucid moses asks a good one. Is the tube connecting the ends of the wormhole meant to be in 4D
spacetime, or somewhere else? The answer is ... somewhere else. The tube Lucid refers
to is in the so-called embedding diagram of the wormhole. It's really a 2-D slice out
of 4-D spacetime, with time and one dimension of space discarded. But the diagram has 3
dimensions, so what is that 3rd dimension? It represents the amount of stretching of the fabric of
spacetime - the strenght of the gravitational field, if you will. Same as with the rubber sheet
analogy in which a massive body depresses the sheet. What "dimension" is the sheet depressing
into? None - it's a representation for the curvature of spacetime.
Lala Fafa asks why the wormhole tunnel is thought to be shorter than the normal-space
path between wormhole ends. Well the simplest answer is "because it can be".
You can write down a solution to the Einstein equations in which the wormhole throat is
short, and it has no relationship to the actual distance between the ends. So you build a wormhole with a short
throat and then move its ends far apart - that doesn't change the lengh of the throat, so
there's your shortcut. In the case of the Schwarzschild wormhole, the distance has to
be short. When the wormhole is fully open, the event horizons of the wormholes intersect.
Pass thorugh one event horizon at that instant you exit the other.
Although New Message puts it into perspective. Fast Interstellar Travel? I'd settle for a
leisurely trip to the grocery store at this point of the lockdown.
Despite the absolutely fascinating topic of wormholes, most of the comment questions were about the cat
that walked through the background during the last comment responses. Now many of you
were properly introduced to Simone during last week's livestream - if not you can go
meet her there, it's still avaialble to watch. But anyway, yeah, that's Simone. She loves
crawling into boxes whenever possible, I assume to test quantum theory. Miraculously emerging
definitely alive every time. She's become a pretty hardcore Everetian always trying to convince me of the
reality of the quantum multiverse and how there are way more treats in other branches of the wavefunction.