In 1998, the Nobel laureate, Brian Schmidt, revealed that the
universe’s expansion rate was accelerating over time 1. This discovery completely
revolutionised cosmological models of the universe, revealed a mysterious force
called dark energy, and also served as a source of personal inspiration. In
order to comprehend how Dr Schmidt accomplished this, one must first understand
the expansion of the universe and Type 1A Supernovae.
An Expanding Universe
In 1929, Edwin Hubble decided to look at the redshift of light
sources in galaxies outside the Milky Way 2. In physics, redshift is a phenomenon
that occurs when a light source is moving away from the observer. It causes the light
waves to stretch out and shift towards the red
end of the visible spectrum 3.
Now as most scientists do, Hubble recorded his
data so that he could create a graph with the following two variables plotted against
The distance of the galaxies from Earth in
parsecs on the x-axis. One parsec is approximately 30 trillion km 4.
The velocity of the galaxies in km/s on the
y-axis. Velocity can be calculated from each light source’s redshift 2.
Figure 1: Velocity-Distance Relation among
Extra-Galactic Nebulae 2
As evident in Figure 1, Hubble’s graph showed the following trend: on average,
the further away a galaxy was, the faster it was moving away.
led Hubble to an inescapable conclusion: The cosmos
was expanding 2.
Since not everyone is an astrophysicist, let’s use a balloon to explain how
Hubble came to this verdict.
In this model, the balloon is the universe, the
black dot (B) is the Milky Way galaxy and the two other dots are neighbouring
galaxies. Now, as the balloon expands from Figure 2 to Figure 3, both Galaxy G
(Green dot) and Galaxy R (Red dot) move away from the Milky Way at the same
Galaxy G travels away from the Milky Way much
more than Galaxy R (Red dot) does. This must mean that Galaxy R is travelling
away faster than Galaxy G.
Figure 3: Inflated Balloon
Representing Universe after Expansion
Figure 2: Deflated Balloon
Representing a Universe
*No references needed for the above
figures as they were created by author.
This exact analogy can be extrapolated to what
Hubble saw. Since further galaxies were travelling away at faster speeds, he
knew that just like the expanding space between the dots on a balloon, the
space between galaxies in our universe was also expanding.
Creating the Perfect Experiment Using Type 1A Supernovae
Using Hubble’s discovery as the foundations, Brian Schmidt designed
an experiment in the early 1990s that set out to determine the ultimate fate of
“Would the universe’s expansion rate eventually change or was it going to stay
the same forever?”
In order to solve this question, Dr. Schmidt needed
to compare the expansion rates of the universe at different points in time. To
do this, he had to collect data on:
The redshift and distance of light sources that were nearby.
This information would enable him to calculate the current expansion rate and
the expansion rate of the relatively recent past 5,6.
The redshift and distance of light sources that were extremely far away.
Since light takes time to travel, looking at very distant light sources enables
us to see what happened long ago in the past.
For example, SN Wilson was a supernova that first appeared in the night
sky in 2013. However, it took 10 billion years for the light from that
explosion to travel to Earth 7. This means that an astronomer who saw this supernova
in 2013 would’ve technically been looking at something that occurred 10 billion
Therefore, one can look at very distant light
sources to obtain data about the expansionary rate of the early universe 5,6.
Recognising that he had to collect data on the
redshift, distance and age of light sources, the next question for Dr. Schmidt became,
“Given that there are so many different sources of light in the universe, which
one is the most suitable for the experiment?”
Well after some careful consideration, Brian decided
that Type 1A Supernovae were the right candidates for this role. These
supernovae occur when white dwarf stars – the extremely dense carbon remains of
a low or medium mass star – start siphoning off material from a nearby
companion star 8,9. Once the white dwarf star is approximately 1.4 times the
mass of our sun, a nuclear reaction occurs, and the white dwarf explodes 8,9.
Since the amount of energy and intrinsic light released from all Type 1A Supernovae
are relatively similar, these explosions can be measured and standardised to
accurately provide all the data that Dr. Schmidt required 5,6 & 8.
Thus in 1994, Brian assembled a team whose role
was to measure the universe’s expansion rate over time using Type 1a Supernovae –
the High-Z Supernova Search Team was born.
Discovering the Universe’s Accelerating Rate of Expansion
search for Type 1A Supernovae was conducted in the Cerro Tololo Inter-American
Observatory (CTIO) in Chile during the 1990s 10,11. Given the level of technology and software at the time, finding
suitable candidates was a lot more difficult and time-intensive than it is
Using the CTIO’s telescopes, they took many
images of the night sky and ran them through Schmidt’s software to find
supernovae that the High-Z team could use for their data analysis 5,11. If the
High-Z team found any candidates, they would then send them to the largest
telescopes in the world at the time – the Keck telescopes in the US 10,11.
Using the computers here, the redshift, distance and age of the nominated supernovae
were calculated 6,10. Now, this type of data collection occurred for several
years until Adam Riess (one of the scientists that Schmidt shared the same
Nobel Prize with) provided Brian with Figure 4 in 1997.
Figure 4: Changes in the Expansion rate of the Universe
Across Time 6
For Figure 4, note that:
– “Redshift” on the bottom x-axis can be interpreted as “time” on the top
x-axis. This is because: The greater the
amount of redshift à The longer the look back time à Hence, the longer ago the supernova 5,12
– The expansion rate of the universe during the 1990s is implied as the benchmark
for the percentage change in cosmic expansion rates 5.
Now, notice that in Figure 4 there are 40
supernovae – 31 in the first circle and 9 in the second. Each supernova is represented
by a dot and an error bar that delineates where the correct answer lies 63.8%
of the time 6. When one looks at the supernovae that exploded relatively
recently (those in the first circle), it is very difficult to identify any definitive
trends. However, if we look at the very distant objects that enable us to see much
further into the universe’s past (supernovae in the second circle), on average,
the data points lie in the top half of the diagram 5,6. This implies that the
expansion rate many of billions of years ago was actually slower than the
expansion rate during the 1990s.
Even though the High Z team was slightly puzzled
at the time, this was the key piece of information that enabled Schmidt to
conclude that universe’s expansion rate was actually accelerating over time!
Impacts of Brian Schmidt’s Discovery
Although Dr. Schmidt’s findings are not yet prevalent in
everyday life, they have had a major impact on cosmological models, dark
energy, and even become a source of personal inspiration.
Prior the experiment, most people, Dr. Schmidt included, believed that the
universe was experiencing one of following two scenarios:
Heavy Universe (a.k.a Gravity wins): In a heavy
universe that is filled with a lot of matter, the gravity between galaxies is
very strong 13,14. As illustrated on the left side of Figure 5, this strong
attracting force would cause the expansion rate to rapidly slow down, stop and
reverse 13,14,15. This would ultimately result in the gnaB giB – the Big Bang
backwards (a.k.a Big Crunch) 16. This scenario is analogous to throwing a
ball straight up into the air. Gravity will slow the ball down over time until
it stops in mid-air. It will then fall back down to the original place that it
was thrown from. Now if we actually lived in a heavy universe, then the second
circle in Figure 4 would have been in the purple area 6. This would’ve
indicated that the expansion rate was decelerating rapidly enough for gnaB giB
Light Universe (a.k.a Gravity loses): In a light
universe, there is enough material to slow the expansion, but not enough to
completely stop it. Eventually, each galaxy will expand far enough from one
another that they will no longer be affected by each other’s gravitational pull
13,14. As illustrated by the model in the middle of Figure 5, this will then
result in the universe expanding at a constant rate for an indefinite amount of
One can imagine this situation as a ball being thrown up all the way from Earth
into space. Although gravity would initially slow the ball down, once it escaped
Earth’s pull, it would continue to travel in the vacuum of space at a constant
rate until all the stars eventually died 16. Now if we actually lived in a
light universe, then the second circle in Figure 4 would have been in the
yellow area 6. This would’ve demonstrated that the expansion was slowing down
over time, but not at a rate fast enough to completely stop universal expansion.
Figure 5: Models of the Heavy, Light and
Accelerating Universe Over Time 15,17
Since the general consensus during the 1990s was
that one of these two scenarios were correct, Schmidt’s new model of an
accelerating universe initially stunned the much of the scientific community 18.
Over time however, his discovery not only led to the abandonment of the
incorrect heavy and light universe scenarios, but also rapidly reshaped cosmological
perspectives about the universe’s future. Many scientists now create theories
on the fate of the universe based on Brian’s accelerating expansion model
rather than the old decelerating scenarios 19. One example of such is the Big
Rip. It hypothesises that the expansion of the universe will become so fast, it
will literally tear its own atoms apart 16,20.
Finding Dark Energy
The realization that the universe was accelerating also led to the discovery of
a new type of energy that scientists were previously unaware of – dark energy
After Schmidt and Adam Reiss (the
corecipient of the Nobel Prize that was mentioned earlier) had figured out that
the expansion rate was increasing and that approximately
70% of the universe’s mass was responsible for this, they hypothesised
the idea that this mass was some kind of energy that pushes faster than gravity
pull 13,21. This was the moment that led to the discovery of dark energy.
Even to this day, dark energy remains a confusing
and mysterious topic for cosmologists and physicists 22. However, because it is
the largest constituent of the universe, it is very likely that neither the
fate nor the origin of the universe can ever be determined without
understanding dark energy first 13. Currently, scientists are attempting to
better comprehend the nature of dark energy using missions
such as Euclid 17,23. This is a launch planned for 2020 that will map out two
billion galaxies with unparalleled accuracy 23, 24. This will hopefully
provide scientists with enough data to determine the exact nature of dark
energy and study its effect on galaxies.
Thanks to the High-Z team’s discovery, missions like Euclid – which attempt to
uncover more information about dark energy – will increase in frequency and
importance over time.
Source of Personal Inspiration
Having always been a strong advocate of
education, I know that learning and teaching aren’t tasks that are always easy.
However, Dr. Schmidt’s endeavours at educating the public on his perplexing
discovery using TEDx talks, lectures and free astronomy courses 6,14,25 encourage
me to continue using my own experiences and knowledge to help others.
Furthermore, during the High-Z team’s research, a competing team called the
Supernova Cosmology Project (SCP) was also working on the same goal 10.
However, rather than antagonizing them, Schmidt happily embraced the rivalry
saying that, “Science benefited from the competition” 6. This attitude of
potentially having to sacrifice individual reward in pursuit for the greater
good is another reason why I truly do admire Schmidt’s character and contribution
In summary, Brian Schmidt’s
utilisation of Hubble’s findings and Type 1A supernovae enabled him to discover
the universe’s accelerating rate of expansion. Over time, this has radically
reformed perceptions of the universe’s timeline, helped to propagate ideas on
dark energy and inspired me personally. As civilisation continues to progress
and a greater understanding of dark energy hopefully emerges, Dr. Schmidt’s
findings will most likely begin to have an even greater impact on society.