25 years studying the Big Bang’s afterglow

This week marks the 25th anniversary
of a landmark discovery in Cosmology.
On the 23rd of April 1992
NASA unveiled a picture painted in microwave light.
This was the first image
of the cosmic microwave background:
the afterglow of the Big Bang.
It revealed what the Universe looked like
shortly after its birth:
the Universe’s ‘baby picture’.
John Mather was one of the key astrophysicists
behind this image.
His interest in the microwave background
went back to his student years
when he was hunting for a thesis topic.
John Mather: "And I looked around and I found a project
which was to build an apparatus
to go up on a high altitude balloon,
so that was my main thesis project.
And we built it and it did not work
and then I got a job at NASA
so I said to my new boss,
well you know my thesis project didn’t work
but we should try it in outer space so that became
the cosmic background explorer satellite."
The microwave background had first been spotted
back in 1964.
But no one had been able
to precisely measure it,
let alone map how it varied across the sky.
This new satellite, COBE
– the cosmic microwave background explorer –
would shed new light on its properties.
But developing COBE took fifteen years,
and huge efforts.
John Mather: "The COBE Satellite took about 1500 people to build.
When we started the COBE mission
people still were designing NASA satellites
with pencils and pieces of paper."
But all this work paid off.
The first major discovery from COBE
arrived just weeks after launch,
in early 1990.
It was not yet a map,
but an analysis of the bands of light
– the spectrum – that make up the radiation.
John distinctly remembers the reaction he got
when he first showed this graph.
John Mather: "When I put up my view graph,
that showed the spectrum
of this cosmic background radiation,
I didn’t say anything and within moments
the entire audience had stood up and was applauding,
so I got a standing ovation for a chart."
Until then, cosmology had earned itself
a bit of a bad reputation.
There were plenty of theories,
but not much data to back them up.
This chart was different.
The data returned by the satellite
was indistinguishable from the theoretical predictions,
but COBE’s most astonishing finding was yet to come.
John Mather: "Our second great discovery
was that the Universe has hot and cold spots.
In Greek we call them anisotropy."
These hot and cold spots
were first shown to the world 25 years ago
by John's colleague, George Smoot.
COBE measured variations in the temperature
of the microwave background across the sky.
The result? A baby picture of the universe,
when it was a mere 400,000 years old.
And the image left scientists gasping yet again.
John Mather: "So, Stephen Hawking said, if I remember roughly,
that it was the most important scientific discovery
of the century if not of all time."
The map was so important
because it showed how the primordial broth
of elementary particles
had clumped into denser regions.
Those regions went on to form galaxies and stars
millions of years later.
John and George shared the Nobel Prize
for Physics in 2006
and COBE’s discoveries made an impression
on many young scientists.
Among them was Jan Tauber who had just started a job
at the European Space Agency.
Jan Tauber: "I was a radio astronomer
who had specialised in doing studies of our own galaxy
which is, from a cosmological perspective,
a very nearby object.
And suddenly there was this guy
talking about the very beginning of the Universe.
It made an impact in the sense
that this was clearly big science."
Jan soon started an effort
to build a European follow-up to COBE, called Planck.
Jan Tauber: "Clearly COBE ushered in a new era
where people tried to do better than COBE
and this was one of the first attempts at that."
Missions like Planck could take much sharper
and more detailed pictures than COBE’s.
But they also confirmed COBE’s results.
John Mather: "I was really thrilled
that we got the right answer the first time
because in the places where they could measure
the same that we measured,
the answers agreed beautifully."
Thanks to progress studying
the cosmic microwave background,
Cosmology has made giant leaps.
It has uncovered everything from the Universe’s age
– 13.8 billion years –
to signs of the enigmatic dark matter.
So has the microwave background
given away all its secrets?
John Mather: "There’s still one wonderful secret that we might find.
We’re looking for gravitational waves."
This is the question at the top of cosmologists’ list:
whether the baby picture of the Universe
contains an imprint of gravitational waves.
These ripples could confirm
that for a brief instant
the Universe ballooned from microscopic to cosmic size
– a theory called inflation.
Cosmologists should be able to spot
the imprint of these gravitational waves
by measuring the polarisation of the microwaves;
this is the way the light is ordered together.
John Mather: "So if we could make that measurement,
and if it matches or does not match the predictions,
then we will have learnt something
about those extreme conditions of the early universe."
Planck’s final set of results
is due out before the end of this year.
And they will include a map of polarisation.
So Planck might have a shot at revealing
the signature of gravitational waves.
Here’s Jan again.
Jan Tauber: "Planck is going to make a unique contribution to that
in the sense that it is able to measure this
at the the very largest scales across the whole sky."
If Planck fails to spot this signal,
other teams are hot on its heels,
but even after so much progress
that picture from 25 years ago
is still as magical as ever.
John Mather: "I still love looking at the picture.
I am so thrilled that it has started off
a cosmic exploration
that tells us something about the Big Bang
that we never guessed we could know."

25 years studying the Big Bang’s afterglow

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