Testing Inflation Theory
What a Differences a Fraction of a Degree Makes in the Weather...
Cosmologists are scientists who study the origin and evolution of the Universe. Naturally, they are excited about the launch of MAP because the satellite
will sort out all the fantastic theories they have about where we came from
and where we are going. Some of these theories will be right, and some will
All of these theories have models, or predictions, about what the microwave
background should look like. One way that cosmologist view the microwave
background is through a graph of temperature fluctuations measured within
patches of the sky. The fluctuations will be slight, sometimes only 1/100,000th
of a degree Celsius. And the patches are small too: one degree on the sky
and less. (The entire sky measures 360 degrees, a unit of area or angular
scale, not temperature.)
The Big Bang
is one theory about the origin of the Universe, but it cannot explain why
there is so much brilliant structure. An explosion produces a mess; yet
the Universe sports a wonderful hierarchy of planets, stars, galaxies, galaxy clusters
and even walls of galaxies. Other theories, such as inflation, aim to complement
the Big Bang theory in explaining the world as we see it today.
is an extension of the Big Bang theory in which the Universe expanded from
an atomic scale to a cosmic scale in a fraction of a second, making the Universe
geometrically flat. This rapid expansion, as you can imagine, would leave
its stretch marks on the cosmic microwave background. Inflation is one of
the prime theories that MAP will test.
Inflation (and subsequent theories that complement it) predicts that the
cosmic microwave background will have a series of "bumps" or very specific
peaks in temperature fluctuations at very specific angular scales. The biggest
bump will be at 1 degree on the sky.
Why 1 degree? First, remember that a degree is a physical distance, about
twice the space covered by a full moon, and that inflation makes the Universe
flat -- no funky curves and hidden distances. Next, imagine the moment of
inflation a mere instant after the Big Bang when the Universe was nearly
infinitely dense. Now think of the fog-bound Universe over the next 400,000
years. Inflation sets the conditions for vibrations to wobble through the
fog. Protons, attracted by gravity, would roll towards each other like marbles in a ditch. Photons trying to shine create radiation pressure pushing the protons out. This pushing and shoving in the fog creates what are essentially sound waves.
Sound waves arise from the interaction of
matter and light in the gravity field of
high density regions of the early universe.
(Click on image to enlarge it.)
One degree corresponds to the distance a sound wave could travel
in 400,000 years in a flat Universe. The temperature (and thus density)
differences caused by inflation would be spaced across the sky in one-degree
Picture a triangle. We are measuring one angle. From this we can determine
the length "across" this angle on the sky because we know the speed of
our sound wave and approximately how long it's been traveling (400,000
years). What we are after is the distance that the microwave radiation has
traveled, the distance "out and away" from us, if you will.
A flat Universe determines one distance; a curved universe determine
another. If instruments measuring the cosmic microwave background
don't see a peak temperature fluctuation at one degree, then inflation
is wrong. If the peaks are at two degrees, for example, then space
must be curved like a sphere (much like the Earth). Then matter and
sound can travel a greater distance in 400,000 years.
Inflation has passed this first test. In 1999, a telescope on a mountain
top in Chile first detected this characteristic scale. In 2000, this was
confirmed and measured more accurately by a pair of balloon-borne telescopes
and a telescope operating at the South pole. But there are more hurdles to
clear. Other temperature peaks at even smaller angular scales -- 0.5 to
0.01 -- correspond to other predictions of inflation theory.
This is where MAP comes in. MAP's thoroughness down to angular scales of
0.2 degrees will essentially smooth out the discrepancies among data from
the many recent non-satellite experiments. MAP will look for bump after
bump, with each one -- just like the main temperature fluctuation at one
degree -- saying something very specific about the nature of the Universe.
A comparison of temperature variations on the Earth
with those of the cosmic microwave background.
(Click on image to enlarge it.)