Measuring the Universe: A Brief History
In the 19th century, astronomers and
philosophers battled about the extent of the Universe. Emanuel Kant argued
that we lived in a galaxy like the faint nebulae seen through telescope,
and he was right, a half a century before it became generally accepted.
The first good look at galaxies
was taken by Vesto Slipher who measured their velocities using the Doppler
shift on spectral lines in the object's spectra. He found that almost every
galaxy was moving away from us! Want
to know more about spectra and the Doppler shift?
Slipher's represented a cosmic conundrum for astronomers of the day:
Since the time of Copernicus, astronomy has presumed that we are not a
special place in the Universe. But Slipher's results seemingly contradicted
this belief - we were a special place, the most unpopular place in the
Universe from which all other objects were trying to move away. Slipher's
results remained a mystery until Edwin Hubble came along in the 1920s with
the then world's most powerful telescope, the recently completed 100inch
telescope on Mt. Wilson, near Los Angeles. He used the physical law that
an object becomes fainter as its distance increases to gauge the distances
to Slipher's galaxies.
 
In 1929 Hubble announced his results. He assumed that the brightest
stars he could see in a galaxy were all the same brightness, and found
that the faster an object was moving away, the fainter its brightest stars
were, thereby showing that the more distant an object, the faster it was
moving away from us. He announced that the Universe was expanding.
Want
to know more about how Hubble's observations indicate we are in an Expanding
Universe?
We name the rate that the Universe is expanding after Hubble -
The Hubble Constant. The Hubble constant tells us how fast an object is
moving away from us, given its distance. If you think about it, the Hubble
constant therefore tells us given an object's distance, how far apart those
two objects will be at any time. If we extrapolate to the time when these
two objects were on top of each other, we reach the big bang. So the Hubble
constant tells us how old the Universe is.
But here we have assumed the Universe hasn't changed in its rate of
expansion over time.
The Changing Speed of the Universe
As the Universe expands, gravity pulls on the Universe, and slows the expansion
down over time. As we look to great distances, we are looking back in time.
If we can measure how fast the Universe is expanding in the past, and compare
it to how fast it is expanding now, we can see the total gravitational
effect of all matter in the Universe.
Here we plot the distance between two galaxies as a function of time.
Looking back into the past we see that the galaxies get closer together
until they are ontop of each other - this is the time of the Big Bang.
If the Universe expands at the same rate, it will follow the dotted yellow
path. But if it is slowing down over time the Universe is younger than
we would otherwise think, speeding up, then it is older.
If there is lots of material, the Universe will be expanding much faster
in the past --- it will have slowed down a lot --- so much so that the
Universe will eventually halt in its expansion, start to contract, and
eventually end in the gnaB giB (that is the Big Bang backwards). In most
models of the universe, this type of Universe curves onto itself (like
a sphere), and is finite.
If there isn't much material, the Universe will be expanding about the
same speed in the past as now, and will continue to expand forever. This
type of universe curves away from itself (like a saddle), and therefore
is without end, now, in the future, and even at the time of the Big Bang.
Here we once again plot the distance between two galaxies as a function
of time. Looking into the future we see that the galaxies get further and
further apart, except if gravity is able to halt the expansion. If the
Universe expands at the same rate, it will follow the dotted yellow path.
But if it is slowing down over time the Universe it eventually turns around
and starts to contract. If the Universe is speeding up, it will continue
to do so at an ever increasing rate.
A favourite model amongst theorists is for the Universe to be precariously
balanced between being finite and infinite. This balanced Universe is known
as a critical universe. Space neither curves away nor onto itself, it is
flat, and is, for most theorists infinite. If the Universe is made up of
normal gravitating matter, it will slow down in its expansion over time,
however, if the Universe has some other forms of material in it, it can
actually accelerate over time.
2D representations of the shape of the Universe. Universes with lots
of material curve onto themselves like a sphere. Universes with little
matter curve away from themselves like a saddle. And Universes with just
the right amount of material are flat.
In practice, we use Einstein's equations of General Relativity to understand
what we see in the Universe. In addition to assuming his theory is right
(it sure seems to be everywhere we have be able to measure so far), we
do have to make a few assumptions. The most important of these are that
the universe is homogenous (that is, the material in the Universe is, on
average, evenly spread through out the Universe) and isotropic (matter,
the expansion, and everything else is the same in all directions that we
look). With these assumptions we can predict how bright an object will
be given its rate of recession (the simple relation found by Hubble breaks
down at large distances).
So, if we can measure distances, we can see how these compare to the
predictions of General Relativity, and in this way we can see what is in
the Universe, and gauge how this material affects the Universe. It turns
out this also allows us to predict what the future holds for the Universe.
Using Type Ia Supernovae to Measure Distances
There are two major types of exploding stars which astronomers label "Supernovae".
Massive stars that explode when they run out of nuclear fuel, and white
dwarf stars that are ignited into gigantic explosions when they acquire
material from a neighbouring star, and exceed 1.4 times the mass of the
sun. These Type Ia supernovae are the key to measuring the distant Universe.
Want
to know more about supernovae?
SN 1994D observed with the Hubble Space Telescope. The SN is the
bright star in the lower left corner.
Type Ia supernovae in the nearby universe are observed to all have a
similar brightness, and this makes them very powerful objects for measuring
distances. By simply observing how bright they are, you can measure how
far away they must be - the further they are the fainter they appear. In
addition, because they are so bright, they can be seen at great distances,
and these two things make them currently unique objects for measuring the
vast distances of the Universe. Unfortunately, they are very rare. The
last one seen in our galaxy was in 1006, and it must have been incredibly
bright - easily visible in the daytime.
The idea to measure the Universe with Supernovae is not new, it has
long been contemplated, but it is only in the past decade that it has become
feasible. The first distant SN Ia was discovered in 1988 by a Danish team,
but it wasn't until 1994 that they were discovered in large numbers. Since
1995 two teams have been discovering these objects: A team which I lead,
the High-Z SN Search, and a team lead by Saul Perlmutter known as the Supernova
Cosmology Project.
To measure the fate of the Universe, we need both distant and nearby
objects, as it is only through the comparison of nearby and distant objects
that the Universe's behaviour uncovered. Amazingly enough, the first good
nearby sample was only completed in 1996 by a group at Cerro Tololo Inter-American
Observatory (CTIO), and these objects are what enable us to use supernovae
to measure the ultimate fate of the Universe.
In actuality, type Ia Supernovae are not all exactly the same brightness.
They vary by as much as a factor of two. But Mark Phillips, Mario Hamuy
and collaborators at Cerro Tololo Inter- American Observatory in Chile
showed that faint supernovae rise and fall very quickly, whereas bright
supernova brighten and fade much more slowly, By looking at how much the
objects faded in the first 15 days following maximum light, their work
showed that type Ia supernovae can give distances which are good to about
7% - equal to the best of astronomical distance indicators.
Discovering the Accelerating Universe
In 1995 my team discovered our first distant Type Ia supernova, SN 1995K.
It was surprisingly faint - so faint that it seemed the Universe was not
being slowed down much by gravity. But it was not until 1998 that we became
convinced something was truly amok with our assumption that the Universe
was filled only with gravitating matter.
Want
to know more about discovering distant supernovae?
In 1998, our team published the figure below.
This plot shows our supernova data. Across the Bottom axis is Distance
(or equivalently time since the SN exploded). The side axis shows change
in the Expansion rate from its current rate.
As you can see the points do not show the Universe slowing down
enough to have a gnaB giB. Furthermore, they do not even seem to show the
Universe coasting, rather they show that the Universe is accelerating!
Every point lies in the bluish green accelerating part of the diagram.
Simultaneously, the Berkeley Group, led by Saul Perlmutter, published data
from which they came to the same conclusion!
Dark Energy
So what could make the Universe accelerate? It turns out one possibility
is a form of Dark Energy invented by Einstein in the 1920s to make sure
his equations of General Relativity did not predict the Expansion of the
Universe. Although Einstein was willing to predict several things with
his equations of General Relativity, he obviously thought an expanding
Universe was too much, and so he invented the Cosmological Constant, a
form of Dark Energy that repels itself, and was meant to balance the force
of gravity, and make the Universe static. He later called this his greatest
blunder once it was discovered, in 1929, that the Universe was expanding
But starting in the 1980s, the Cosmological Constant has made
resurgence as a cosmic panacea. Although the Cosmological constant seems
capable of curing many of the ills of the modern cosmology such as the
age problem: (A Universe with a lot of cosmological constant is older than
we might otherwise think, therefore making the age from the Hubble Constant
and the age of the oldest stars in agreement).
Here we show how much cosmological constant and normal matter (that
is the stuff you and I and everything around us that we see is made out
of) there is in the Universe. The darker the color, the more likely a particular
combination of the two types of matter is. The numbers are plotted as the
greek letter Omega which is the symbol we use to represent the fraction
of the Universe made out of matter relative to the amount necessary to
make space flat. (i.e., if Omega = 1 then there is just enough matter to
make the Universe flat). As you can see, it appears that the total amount
of normal matter in the Universe is quite small - Omega is less than 1
for normal matter, and the amount of cosmological constant is reasonably
large - Omega greater than 0 for the cosmological constant.
Supernovae are not the only way to make such a diagram. Another way
is to look at the Universe when it was very young, and glowed like the
sun. This radiation is the furthest thing we can see in the Universe, and
is known as the Cosmic Microwave Background. By observing how the lumps
and bumps in this background are distributed, it is possible to measure
the amount of matter in the Universe. New measurements from this method
give their own measurement of Matter and Cosmological Constant.
If we combine the two measurement into one diagram, we can figure
out what both methods together are telling us right now. As you can see
the best values (the hashed red and white area where the two methods overlap)
are having normal matter 0.2 of the critical value, and the cosmological
constant about 0.8 of the critical value.
This is actually quite amazing because many theorists want the sum of
Omegas in all types of matter to be one - this is the prediction of a theory
called Inflation - and these results suggest that it just might be.
As mentioned earlier, we are one of two groups working on this project,
and we have arrived, using different data, to almost identical conclusions
at the same time, without knowledge of each other's results. The above
diagrams show both teams data (they are the Supernova Cosmology Project).
But just because we get the same answer does not mean we are both right,
we could both be being fooled by our supernovae in the same way. Or maybe
our whole cosmological model is flawed.
So these observations may really mean one of three things:
The Exciting: the Universe is accelerating. The Universe is accelerated
by some unknown type of matter that is spread throughout the Cosmos.
The Heretical: General Relativity is as sacred as anything in Physics,
but it may be wrong. Since our work is comparing the predictions of General
Relativity with observations, if General Relativity is wrong, so are our
conclusions.
The Mundane (at least from our point of view): We are simply wrong
and have been fooled by Supernovae into believing the Universe is accelerating.
Maybe supernovae are fainter in the past, and therefore appear further
away then they really are.
We hope and believe it is the first alternative, but we have to work
hard and test to see if it isn't the second or third alternative. The accelerating
Universe is a major revolution in our understanding of the Universe around
us. While the evidence is now strongly in favour of this conclusion, extraordinary
claims require extraordinary proof.
Dark Energy Versus Gravity
Despite other methods, Supernovae are currently the only viable method
to actually measure the Universe is accelerating. But Einstein's version
of Dark Energy makes a strong prediction, it should have been much weaker
in Universe's past.
Like two wrestler's vying for domination of the Universe, Dark Energy and
Matter were then poised against each other in a fight for the future of
the Universe 7 Billion years ago. Dark Energy pushes the Universe apart,
while Matter pulls it together. By looking back even further, instead of
to 5 Billion years, to 10 Billion years in the past, it is possible to
see this battle when gravity was much stronger than it is currently. If
Einstein's Dark Energy is the cause of the acceleration, it should have
overwhelmed Gravity 7 Billion years ago - before this time, Gravity was
winning the war, and slowing the Universe down.
Unfortunately, supernovae are very faint at distances beyond 7 Billion
light years. In 1995 the deepest map of the sky was made by the Hubble
Space Telescope called the Hubble Deep field. In 1997, this field was reobserved
by two astronomers, Gilliland and Phillips, and a very faint supernova
was discovered, called 1997ff (See figure)
The Hubble Deep Field showing the thousands of galaxies observed
in 1995. In 1997, SN 1997ff, exploded in one of these many galaxies (left
bottom panel), revealed by subtracting the 1995 data from the 1997 data
(right panel)
By itself, this discovery image was useless. How far was the object,
how old was the object, how fast was the object moving away? were all questions
that needed to be answered. But this object was too faint to be observed
with anything other than the Hubble Space Telescope. Furthermore, the observation,
rather than being made in optical light, needed to be made in the infrared,
because the doppler shift, at great distances, shift the light we used
to study the supernova so much, that this light ends up stretching outside
the visible region. Late last year it was realised by Adam Riess, a colleague
of mine who works at the Space Telescope Science Institute, that a number
of infrared observations were made of a tiny region of sky near the position
of SN 1997ff, for a completely different purpose, just at the time this
object had exploded. This serendipitous data set was just what we needed
to measure the distance to this object.
For six months we studied this data set to sift how many photons had arrived
from SN 1997ff at each colour. These data showed that SN 1997ff was at
a redshift of z=1.7 (meaning its light had been shifted by 170%), and was
more than 10 billion light years in distance. These observations allowed
us to measure how fast the Universe was expanding 10 billion years ago,
and this most distant exploding star provided the remarkable answer. As
predicted, the Universe was slowing down 10 billion years ago, just as
Einstein's equations predict.
The Future
As the Universe accelerates, galaxies are getting further apart faster
and faster. Assuming this continues, the inescapable conclusion is that
the expansion of space between us and galaxies we see now will eventually
exceed the ability of light to reach us. Eventually, in the hundreds of
Billions of years in the future, this will be true for the entire Universe
outside of the nearest few galaxies. This stretching of space beyond the
speed of light is not true for places where there is currently enough gravity
to cause the Universe to collapse - here the Dark Energy can never take
hold a stretch the Universe to infinity. So our galaxy will be spared,
but we will look out onto a cold, dark and empty Universe from our vantage
point in the Milky Way.
But this is bleak prediction of the Universe's future is predicated
on the idea that the acceleration will continue (which is true if Einstein's
Cosmological Constant is the culprit). But other theories exist with names
such as quintessence, x-matter, and Vamps. These theories allow the Universe
to almost anything in the future, but can be tested to a reasonable degree,
by very careful measurement of the Universe's past. Is the rate of acceleration
exactly as predicted by Einstein's Cosmological Constant, or is it different.
Supernovae maybe able to provide an answer - provided the difference is
not too subtle - but resolution may require a whole new class of experiments
that astronomers are just beginning to dream up. Only time will tell. |
