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Transcript
Ivelina Sotirova
Major: Biochemistry
Senior at CUNY Hunter College
The Accelerating Universe
The discovery of this year’s Physics Nobel Prize Laureates challenged many concepts
in the field of astronomy as it revealed a secret our Universe had been keeping for billions of
years. The Supernova Cosmology Project (SCP) led by Saul Perlmutter, and the High-z
Supernova Search Team (HZT) led by Brian Schmidt and Adam Riess defied the very status
quo of cosmology when they discovered independently, but concurrently, that, instead of
slowing down, distant galaxies are accelerating further and further away from us.
The evolution of our Universe began 14 billion years ago with an event known as the
Big Bang: an explosion in which time and space were created out of a single point. Energy in
the form of particles propelled through space forming matter and eventually giving birth to
the young Universe. The cosmos began expanding and more and more celestial bodies
formed. But just like Sir Isaac Newton observed the apple falling due to Earth’s gravity,
scientists believed that the mass of the Universe was sufficiently large to slow the expansion
down and, over time, start pulling matter back together. In the 1920’s the Hubble Law was
developed and scientists realized for the first time that the Universe was in fact, after many
billions of years, still growing [3]. The big question then became: Would our Universe really
end up in a big implosion? and, more importantly: If so, when? Physicists became
increasingly interested in finding out the exact rate of deceleration. Solving the equation was
equivalent to foreseeing the future of our Universe. The quest to become the next prophet in
cosmology began in 1988 with Saul Perlmutter and his team. Inspired by the promising
research, in 1994 Brian Schmidt’s HZT also joined the competition.
The first step in the journey was finding out an accurate measure for the vast cosmic
distances. In principle, this could be done by using certain astronomical objects – a concept
largely based on the work of American astronomer Henrietta Leavitt. In the beginning of the
century her study on Cepheids (a particular type of pulsating stars) brought about the
ingenious idea of standard candles [3]. She showed that, by knowing the intrinsic luminosity
properties of any distinguishable class of celestial bodies, one could measure distances of
hundreds of thousands of light years away. But since Cepheids were only visible on the
smaller cosmic scale, Walter Baade and Fritz Zwicky further elaborated on the idea in 1938
when they discovered an even better standard candle candidate, identifiable over billions of
light years away – supernovae [2].
A supernova is the violent death of a star thousands of times the size of our Sun. As
its fuel gets depleted, the star swells up and grows fainter until it becomes a slow-burning red
giant. When all the hydrogen and helium are used up, gravity disintegrates the star’s outer
layers, leaving behind only the immensely dense hot core – a so called white dwarf. Nuclear
fusion within the core of a white dwarf continues, as heavier and heavier elements are
created. It is now known that when iron starts forming in the core of such star, the energy to
fuse these atoms into heavier elements can no longer be supplied. If the white dwarf is a part
of a binary star system, its strong gravity starts draining gas from its neighbor to supply for its
own deficiency. As the mass of the white dwarf grows to exactly 1.4 solar masses, a nuclear
overload causes its death in a process called a type Ia supernova (SNe Ia) [1]. The white
dwarf becomes so unstable, that in a matter of seconds it explodes, blasting radiation out into
space. It is the single most violent event in the Universe; an explosion so bright it can, over
the course of several weeks, outshine an entire galaxy [3].
In the aftermath, heavier elements such as silver and gold are created. Today scientists
can detect these elements, and classify supernovae according to the difference in their
absorption features. SNe Ia were identified by William Fowler (Nobel Prize in Physics 1983)
as especially interesting because they had no hydrogen features in their spectra but they
E-mail: [email protected]
1
Ivelina Sotirova
Major: Biochemistry
Senior at CUNY Hunter College
did have a silicon absorption line [1]. Even more importantly, they exhibited unique
uniformity in their light curves – the plots of brightness evolution following the explosion
[1]. The validity of supernovae as a reliable marker throughout the deep space was officially
confirmed by later research, and soon they were accepted as the perfect cosmos mapping tool
[1]. Thus, the idea that the expansion of the Universe could be measured by using supernovae
as standard candles was the very stepping stone for SCP and HZT’s groundbreaking research.
Finding SNe Ia however, seemed like a hard task in the early 1980s. Distant
supernovae could not be easily identified with regular telescopes because their emissions
appeared distorted due to the presence of hot plasma. Furthermore, they were found to be
fairly unpredictable, occurring only once or twice every thousand years in a single galaxy [1].
In the rare event when SNe Ia were captured, it was often after their peak brightness [1].
Another serious obstacle physicists stumbled across when studying images of supernovae
was extracting background light from the host galaxy [1]. For a decade scientist scanned the
cosmos in search of SNe Ia with little success. Even though Saul Perlmutter, Brian Schmidt
and colleagues were not the first ones to study SNe Ia in the hopes of solving the presumed
deceleration of the Universe, it was their ingenious logistics that allowed them to record
sufficient and consistent data, rendering the research a triumph.
Despite the rarity of SNe Ia, there are billions of galaxies throughout the Universe.
This led Saul Perlmutter to the conclusion that at least a dozen new supernovae were
happening every day. His decision on looking into the furthest galaxies in search of
supernovae more than 6 billion light years away was not only brilliant, but as it turns out,
pivotal in making the huge discovery. Only supernovae more than one third across the
Universe were studied [3]. In this way any distortions from local light sources would be
minimized, and calculation would be more accurate. At first a 4-m telescope coupled to the
newly developed charged-coupled devices (Willard Boyle and George Smith, Nobel Prize in
Physics 2009) was used to image large batches of sky at once [3]. The evolution of the
supernova brightness happens over the course of two to three weeks requiring at least several
observations during this time. The CCD images guaranteed many new supernovae, but
couldn’t follow up to spectroscopically identify them or record their brightness curve [1]. In
order to do his, they needed to use much more powerful telescopes. Gaining access to them,
however, was no easy task since waiting lists were filled up for months in advance.
Contemplating on how to solve the problem, a second ingenious plan, called Supernova on
Demand, was negotiated with much effort by SCP in 1994 [1]. This method guaranteed prescheduled access to the most powerful telescopes on Earth for more precise observations
during the dark phase of the moon. Once they had these strategies in their arsenal, obtaining
and deciphering data would be more or less straightforward. Prompted by the success of the
research, the High-z Supernova Search Team launched a competing project the same year.
The two teams started working side by side in the observatories in Chile, Hawaii and La
Palma.
The reason for studying supernovae’s brightness was that as light travels, the
expanding Universe stretches not only the wavelength of photons coming through space, but
the very distance between galaxies [1]. During the time interval in which light has traveled to
reach the Earth, its wavelength has been increased by precisely the same incremental factor
by which the cosmos has stretched [1]. The farther the star is, the longer the wavelength of
light coming from it, and the longer the wavelength, the redder its color. This is known as the
redshift, (the z) of an object; a term pioneered by Vesto Slipher in 1912 [2]. The
proportionality between an object’s redshift and its distance was first estimated by Edwin
Hubble [1]. The larger the magnitude of z, the fainter the object and the farther away it was.
Therefore, by measuring the redshift of supernova, one is able to extract exact information
E-mail: [email protected]
2
Ivelina Sotirova
Major: Biochemistry
Senior at CUNY Hunter College
about its remoteness and velocity. The two groups recorded amazingly uniform light curves
for the supernovae, setting up a template to single out any deviations, and for further
necessary calibrations. A collection of recorded redshifts and brightness curves over a
sufficiently large range of distances provided the Nobel Laureates with a measurement of the
total integrated expansion of the Universe [1].
The SCP team used data from a total number of 42 SNe Ia, whereas the HZT team
used data from 16 SNe Ia. After further analysis, both teams found that the stars faded much
faster than predicted by assuming a uniform rate of expansion. Neither team expected such
results, since in a universe which is slowing down, supernovae should appear brighter [3]. By
comparing their almost identical data, both teams were reassured of the validity of their
findings. Supernovae were indeed receding faster and faster from us, and the further they
were, the faster they were drifting away [3]. As if across from the Universe someone had
turned on a vacuum cleaner. Thus, their much awaited results came with a shocking twist.
The parameter presuming deceleration of the Universe had two minus signs in front of it. In
simple words, a minus and a minus is always a plus. So a final and unexpected conclusion
was presented: The Universe was accelerating. The many years of painstakingly observing,
recording, and sorting out reliable data by scanning the skies for small bright dots in the hope
of finding a supernova finally paid off for both teams in 1998 when they officially published
their results.
Today, as a consequence of the SNe Ia research, we know that we live in a spatially
flat [2], low density Universe, where the gravitational mass is not sufficient to stop its
expansion. Small amounts of matter in the vast cosmic space also meant that the proportional
amount of vacuum energy was much larger than previously thought. In fact, data from the
Cosmic Microwave Background research (John Mather and George Smoot, Nobel Prize in
Physics 2006) and the SNe Ia discovery showed that matter as we know it makes about 4% of
the Universe [2]. At the present time scientists agree that dark energy (vacuum energy)
constitutes approximately three quarters of our Universe and is believed to be the very reason
for its acceleration [2]. The remaining one quarter is an even more mysterious substance of
which almost nothing is known – dark matter. The implications for the future of the Universe
are captivating but also frightening. Instead of ending up in a giant galactic collision, like
scientists once dreaded, as time goes by, galaxies will drift farther and farther apart. The
cosmos will grow colder and darker. Space travel will become obsolete in its current terms.
No one knows exactly how our Universe will continue its evolution, and how and when it
will die, but the one thing we do know is that until then, many billions of years will have to
pass. The accelerating Universe is currently a topic of intense research and hopefully with the
help of the SNAP (the supernova/Acceleration Probe) telescope new data will soon uncover
another long-kept secret.
The excitement of finding the unthinkable and for knowing the unknowable has been
driving the scientific curiosity and the research spirit forth since ancient times. In the
beginning of the century Albert Einstein began unraveling the mystery of spacetime with his
General Relativity theory, proving that only with unparalleled imagination are great
discoveries possible. Many questions remain for cosmologists to uncover. What creates dark
energy and dark matter and for what purpose? Is it the imbalance between the two in the
present Universe that is causing it to accelerate? Or is it something even more extraordinary,
such as larger mass distributions further away than we have observed? Hopefully in the near
future more great discoveries will be able to answer these questions and put together the final
pieces of the puzzle called “Our Universe”.
E-mail: [email protected]
3
Ivelina Sotirova
Major: Biochemistry
Senior at CUNY Hunter College
References
1. “Supernovae, Dark Energy, and the Accelerating Universe”. Saul Perlmutter. Supernova
Cosmology Project. 29 Nov 2011.
http://supernova.lbl.gov/PDFs/PhysicsTodayArticle.pdf
2. “The Nobel Prize in Physics 2011 – Scientific Background”. Nobelprize.org. 29 Nov 2011.
http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/advancedphysicsprize2011.pdf
3. “The Nobel Prize in Physics 2011 – Popular Information”. Nobelprize.org. 29 Nov 2011
http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/popularphysicsprize2011.pdf
E-mail: [email protected]
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