Ultimate Fate of the Universe
Nobel Prizes in
Astrophysics & Cosmology - Part 8
(A Twelve Part
Series)
Perlmutter, Schmidt
& Riess
The
missing link in cosmology is the nature of dark matter and dark energy.
-
Stephen Hawking
The Nobel Prize is equated with the pinnacle of human
achievement in both popular perception and professional
esteem. Since it was first awarded in 1901, the annual Nobel Prize
for Physics has gone to major contributions in Astrophysics and Cosmology
related fields only on eleven occasions. The first seven awards (1967, 1974,
1978, 1983, 1993, 2002 and 2006) were the subjects of earlier articles (see
here 1,2,3,4,5,6,7). The next was in 2011, shared by Saul Perlmutter,
Brian Schmidt and Adam Riess for their incredible discovery concerning the
ultimate fate of the Universe.
Einstein's General Theory of Relativity laid the foundation for Cosmology. It is a pleasant coincidence that this article appears on his birthday today.
Background
[By way of a detailed introduction to the nature of our dark Universe, I reproduce below, with minor changes, an article whose Kannada translation appeared in ‘Khagola Darshana’, brought out in 2023 by Navakarnataka Publishers (see here).]
The Nobel Prize in Physics for 2011 was awarded to three cosmologists - Saul Perlmutter, Brian Schmidt and Adam Riess - for their mind-boggling discovery concerning the ultimate fate of the Universe. Their discovery answered one of the most profound questions confronting cosmology and the answer gave no comfort to anyone. Their findings are attributed to the effect of what has come to be called Dark Energy permeating the Universe.
Expanding Universe
Hubble's discovery of distant
galaxies speeding away from us, and from each other, at ever increasing speeds, could be interpreted only one away – that they must have started
off on their journeys close
together once upon a time in the distant past.
The Universe must have originated from a 'primordial atom' marking its
very birth. The entire Universe must have been an incredibly dense, super-hot
and compact entity which, for some as
yet unclear reason, blew up in a gargantuan explosion, started expanding very rapidly and in due course ended up in the
form that we find today. This theory
attracted attention as an alternative to the prevailing 'steady state'
theory which visualized the Universe as without a beginning or an
end and without any mad rush among its inhabitants to run away from each other. Fred Hoyle,
who had championed the steady state theory and was unimpressed by the new idea of an expanding Universe with a definite
beginning in time, called the latter disdainfully as the Big Bang and
the name has stuck ever since.
Einstein's Blunder
The concept of an expanding Universe with a definite beginning had been an unwelcome consequence of the General Theory of Relativity. Einstein had derived an equation for the behavior of the four-dimensional space-time continuum in which gravitation was interpreted as a geometrical property of space, with matter producing a distortion or curvature of it to enable objects to move in the way they are observed to move. An expanding Universe was a logical consequence of such an exercise. However, Einstein's belief in a static steady state Universe was so strong that he tweaked his equation to conform to the concept by arbitrarily introducing an extra term called the cosmological constant into his result. Later, when he realized that the theoretical foundation for the expanding Universe had been staring at his face all the time and that he had lost out on what was perhaps the most important discovery in cosmology up to that time, he termed the introduction of the cosmological constant as the biggest blunder of his life!
Cosmic Microwave Background (CMB)
The observed accelerations of faraway galaxies
did provide a very sound argument for the Big Bang theory but more was needed
before it could supplant the steady state theory and take root as the definitive theory of formation and evolution of the Universe. Such a 'smoking
gun' was provided
serendipitously in 1965 by two researchers in the USA who
were using a horn antenna for detection of annoying radio interferences from nearby sources.
A systematic observation led the two, Arno Penzias
and Robert Wilson,
to the totally unexpected and puzzling discovery of a uniform but faint
microwave background radiation coming
from everywhere in space. The radiation was interpreted as left over from an early
stage in the evolution of the Universe,
and its discovery was considered a crucial test of the Big Bang theory of the Universe. From the measured
wavelength and energy distribution of this cosmic
microwave background (CMB) radiation they could determine the temperature of
the material emitting the radiation to be
around 3K (three degrees on the Kelvin scale, which corresponds
to about -270 0C). With some assumptions, it was possible
to estimate the temperature
at which the whole process should have started and the rate at which the cooling has proceeded since then. The starting
temperature should have been so enormously
high that the 'primeval
atom' should have consisted only of pure energy, with particles materializing from the energy at a
slightly later, cooler epoch. From
then on, the formation of nuclear
matter, synthesis of higher elements, atoms, molecules, etc., should have all followed
according to the laws of physics that are now well
understood.
The Evolving Universe
Our present-day understanding
of the evolving Universe based on the Big Bang model rests on a rock-solid foundation of
observational evidence provided by instruments of ever-increasing
sophistication, particularly the Hubble Space Telescope.
Without pausing to address the crucial question of how, let us summarize what we know about our Universe today. Incidentally, when we talk of an expanding Universe, we mean that space itself is expanding, carrying with it the galaxies like spots on the surface of an inflating balloon, in a manner consistent with Einstein's General Relativity.
The Big Bang event is now reliably determined to be 13.7 billion years old. It all began in a blinding flash lasting the tiniest fraction of a second, smaller than any time duration encountered in any physical process, during which the primordial atom underwent an incredibly large and exponential expansion to set the whole evolutionary process in motion. This has come to be called the inflationary phase.
Structure and Properties of the Universe
Evolution Scenarios
As they reach the end of their life cycles,
super massive stars may use up their nuclear fuel and collapse under their own weight. The collapse leads to an incredibly gigantic explosion, sends out a shock wave through space followed by a shell
of material ejected
at great speeds
from the star's atmosphere. This
is accompanied by the release of enormous amounts of radiation in a blinding flash that lasts a very short period,
followed by rapidly decreasing brightness over a period of just a few weeks.
Called a supernova, this is a relatively rare event within any galaxy, on the average only
one in a hundred years or more. At
its peak, a supernova can outshine
the entire galaxy within
which it occurs.
For this reason,
it is relatively easy to detect and study
a supernova even within the most distant galaxies.
A Type Ia supernova results
from having a white dwarf star in a binary
system. Because of the huge
gravitational force of the dwarf
star, matter is sucked in continuously from the normal star to the white dwarf until the latter
attains a critical mass (the Chandrasekhar limit)
and undergoes a thermonuclear explosion. Because all white
dwarfs achieve the same mass before
exploding, they all achieve
the same luminosity and can be used as standard candles. By observing their apparent brightness, the actual distance
can be determined by using the inverse
square law for brightness
variation with distance.
By knowing the distance to the supernova,
we know how long ago it occurred. By measuring the red shift
from the spectrum
of the supernova, we can determine its
speed of recession and hence how much the Universe has expanded since the explosion using Hubble's law.
Dark Energy
In the 1990's, two teams of astronomers,
working independently but in frequent touch with each other, were at work. One of them was in the Supernova
Cosmology Project based at the Lawrence Berkeley
National Laboratory led by Perlmutter and the other at the (international)
High-Z Supernova Search
Team led by Schmidt in Australia. They investigated a large
number of distant Type Ia
supernovae in order to measure the expansion rate of the Universe. Consistent with widespread belief,
they expected that the expansion
would be slowing
down (according to one of the scenarios A, B or C indicated in the previous
diagram). They expected the supernovae to be actually
brighter than indicated by their measured
red shifts. Instead,
to their utter surprise, bordering
on consternation, they found the supernovae to be actually fainter than expected. The brightness of the supernovae decreased
with distance significantly more rapidly than anticipated. There could only be one interpretation of this. The
expansion of the Universe was accelerating,
as in scenario D of the
diagram!
Evidence for the existence of
dark matter is now clear-cut though its nature is still very elusive. By contrast, dark energy remains
a complete mystery. The name dark energy implies that some kind of 'stuff' must fill the vast reaches of mostly
empty space in the Universe in order to produce an accelerated expansion. In this sense, it is a 'field' just like an electric field.
However, this analogy
breaks down completely when it comes to dark energy.
While bright ideas to account for dark
energy and its effects are not wanting, for the present at least its nature is a mystery
waiting to be unraveled as in any good detective story.
The 4% Universe
Just half a century
ago, we were blissfully ignorant
of the fact that the Universe we were then familiar
with was only about four percent of the 'real' one that we have just begun to recognize. The rest of it was, and still is, hidden from our direct
experience.
The Galloping Universe
Saul Perlmutter (1959 - )
– A Biographical Sketch
Saul Perlmutter was born on 22 September 1959 in Champaign,
into an academically inclined family. His father, Daniel Perlmutter, was a
professor of chemical engineering at the University of Pennsylvania, while his
mother, Felice Perlmutter, worked in community service. He grew up in an
intellectually stimulating environment that encouraged curiosity about science
and the natural world. As a student, he showed strong aptitude in mathematics
and physics.
Perlmutter attended Harvard University, where he completed
his B A in Physics (1981). During his undergraduate years he worked with
Sheldon Glashow, which helped solidify his interest in fundamental physics and
cosmology.
He then moved to the University of California, Berkeley for
graduate study. Under the supervision of Richard A Muller, he earned his PhD in
physics in 1986. His doctoral work involved experimental particle physics, but
he soon turned toward cosmology—an area then undergoing rapid transformation
due to new observational techniques.
Early Career and the Supernova Cosmology Project
After completing his PhD, Perlmutter joined the Lawrence
Berkeley National Laboratory (LBNL) in California. There he initiated one of
the most ambitious observational cosmology programs of the late twentieth
century: the Supernova Cosmology Project.
At the time, cosmologists were trying to determine the fate
of the universe. Two possibilities were widely discussed:
The universe might eventually slow down and recollapse under
gravity. Or it might expand forever, gradually slowing but never reversing. To
measure this, astronomers needed to determine how the expansion rate of the
universe had changed over time.
Perlmutter realized that a special class of stellar
explosions—Type Ia supernova—could serve as “standard candles.” Because these
explosions have nearly uniform intrinsic brightness, their apparent brightness
allows astronomers to determine their distance very accurately.
Measuring Cosmic Expansion
Perlmutter’s team developed innovative techniques to discover
distant supernovae systematically. They used digital imaging and
image-comparison algorithms to detect new supernova explosions in distant
galaxies.
This approach allowed the team to measure the distances to
supernovae billions of light-years away and compare them with the galaxies’
redshifts, which measure how fast the universe is expanding.
In 1998, the Supernova Cosmology Project announced a
startling result: The expansion of the universe is accelerating rather than
slowing down. This meant that some unknown force was pushing space apart.
At nearly the same time, a rival group—the High-Z Supernova
Search Team, led by Brian P Schmidt and Adam G Riess—independently reached the
same conclusion.
Dark Energy and a New Cosmology
The unexpected acceleration implied the existence of a
mysterious component of the universe now called Dark Energy.
Dark energy appears to: permeate all of space, exert a
repulsive gravitational effect and dominate the energy content of the universe.
Current cosmological measurements suggest that: ~5% of the
universe is ordinary matter, ~27% is dark matter and ~68% is dark energy.
Perlmutter’s discovery profoundly altered modern cosmology
and revived interest in Cosmological constant, originally introduced by Albert
Einstein.
Nobel Prize and Recognition
For this discovery, Saul Perlmutter, Brian Schmidt, and Adam
Riess were jointly awarded the Nobel Prize in Physics.
The Nobel citation recognized: “the discovery of the
accelerating expansion of the Universe through observations of distant
supernovae.”
The finding is widely regarded as one of the most important
discoveries in cosmology since the detection of the Cosmic Microwave
Background, first observed by Arno Penzias and Robert Wilson in 1965.
Later Work and Scientific Leadership
Perlmutter continues to work at Lawrence Berkeley National
Laboratory and teaches at the University of California, Berkeley.
His later work focuses on: precision cosmology, improved
measurements of dark energy, large sky surveys and the statistical analysis of
cosmological data.
Scientific Style and Legacy
Perlmutter’s work is notable for combining careful
experimental technique with ambitious cosmological questions. His group
pioneered large collaborative observational programs in astronomy—an approach
similar to particle-physics experiments.
The discovery of cosmic acceleration transformed cosmology
from a largely theoretical discipline into a precision observational science.
Today, the question raised by his work remains one of the
deepest in physics: What exactly is dark energy? Answering this question may
require new physics beyond current theories of gravity and quantum fields.
Adam G Riess (1969 - ) – A Biographical Sketch
Adam Guy Riess was born on 16 December 1969 in Washington, D C.
He grew up in Warren Township, where his early interest in science and
mathematics became evident during his school years.
He studied physics at the Massachusetts Institute of
Technology, graduating in 1992. During this period, he developed a strong
interest in observational cosmology and astrophysics.
Riess pursued graduate work at Harvard University, completing
his PhD in 1996 under the supervision of Robert P Kirshner, a leading authority
on supernova research. His doctoral work introduced improved methods for using
Type Ia supernova as accurate distance indicators.
The High-Z Supernova Search Team
After his doctorate, Riess became a key member of the High-Z
Supernova Search Team, led by Brian Schmidt. Riess played a central role in the
data analysis and calibration techniques used to determine the luminosity of
distant supernovae.
In 1998, Riess led the team that published one of the most
important papers in modern cosmology. The analysis showed that distant
supernovae were fainter than expected, implying that they were farther away
than predicted by a decelerating universe.
The only consistent interpretation was that the expansion of
the universe is accelerating.
Later Scientific Contributions
Riess later used the Hubble Space Telescope to refine
measurements of the Hubble constant, which describes how fast the universe
expands.
His precise measurements have led to what is now known as the
“Hubble tension”—a discrepancy between the expansion rate measured from the
early universe (from the Planck), and the expansion rate measured in the nearby
universe using supernovae.
This tension may hint at new physics beyond the standard
cosmological model.
Riess currently works at the Space Telescope Science
Institute and Johns Hopkins University.
Nobel Prize
In 2011, Riess shared the Nobel Prize in Physics with Saul
Perlmutter and Brian Schmidt for discovering the accelerating expansion of the
universe.
Notably, Riess was only 41 years old at the time—one of the
younger Nobel laureates in physics in recent decades.
Brian P Schmidt (1967 - ) – A
Biographical Sketch
Brian Paul Schmidt was born
on 24 February 1967 in Missoula. His family later moved to Anchorage, where he
spent much of his childhood. As a student he developed a fascination for
astronomy and physics, often building experimental equipment and conducting
amateur observations.
Education
Schmidt studied physics and
astronomy at the University of Arizona, receiving his undergraduate degree in
1989.
He then moved to Harvard
University for doctoral research under the supervision of Robert Kirshner,
earning his PhD in 1993. His doctoral work focused on supernovae and
cosmological measurements.
Formation of the
High-Z Supernova Search Team
After completing his PhD,
Schmidt moved to Australia and joined the Australian National University,
working at the Mount Stromlo Observatory.
There he organized the
High-Z Supernova Search Team, an international collaboration designed to
discover and study distant supernovae.
Schmidt coordinated the
observational campaign while Adam Riess led much of the data analysis. Their
measurements confirmed that distant supernovae appeared dimmer than
expected—evidence for accelerated cosmic expansion.
Scientific Leadership
Following the Nobel Prize,
Schmidt became one of the most influential science leaders in Australia. In
2016, he was appointed Vice-Chancellor of the Australian National University,
effectively serving as the university’s president.
He has continued to advocate
for: large international astronomical collaborations, public science education,
and long-term investment in fundamental research.
A Historic
Convergence in Cosmology
The work of these three
scientists—Perlmutter, Schmidt, and Riess—forms one of the most striking
examples of independent confirmation in modern science.
Two competing teams using
different datasets and methods arrived at the same unexpected conclusion: The
universe is dominated by a mysterious component now known as dark energy.
Today this discovery
underpins the standard cosmological model (ΛCDM) and remains one of the deepest
unsolved problems in physics.
The Ultimate Fate of the Universe
The Earth is an insignificant part of the solar family that occupies an equally insignificant place in the Milky Way galaxy. Less than a century ago, we discovered that this is itself an utterly insignificant part of the Universe as we came to know. Now we know that the Universe we have been able to understand so far is, yet again, an insignificant part of the one that we are yet to understand. This sums up not only the level of our ignorance but also the degree of our insignificance. The vast Universe is still crying to be understood.
However, we do seem to have at this time a plausible answer to one of the ultimate questions bothering humankind for centuries – what is the ultimate
fate of the Universe? Cosmology
is no longer an
infant discipline depending on philosophy and metaphysics to provide the answers.
It is now a full-blown science that
can face this question head on.
Hubble Trouble
Right now, we seem to be having some highly irritating trouble with the Hubble constant that describes how fast the universe is expanding at different distances from Earth. Measurements of the rate of cosmic expansion of far off galaxies using different methods keep turning up disagreeing results, implying that Hubble constant may not be a constant after all and its value depends on how it is measured. Using data from the European Space Agency's (ESA) Planck satellite, scientists estimate the rate of expansion to be 67.4 km/s/Mpc. However, calculations using the pulsating Cepheid stars suggest it is 73.4 km/s/Mpc. As of now, these values are irreconcilably different and sufficient to sow the seeds of a crisis in astrophysics and cosmology. With the nature of dark matter and dark energy, which constitute nearly 96% of the Universe, still an unsolved puzzle, this discrepancy adds further fuel to the fire brewing in our endeavor to understand the universe.
Current Status
ChatGPT summarizes the current status of dark energy research as follows:
Recent observations—especially from the Dark Energy Spectroscopic
Instrument (DESI)—have provided the most precise measurements yet of the
universe’s large-scale structure and expansion history. By mapping tens of
millions of galaxies and quasars across billions of years of cosmic time, DESI
has strengthened earlier evidence that the universe is expanding at an
accelerating rate due to Dark Energy. However, when DESI results are
combined with supernova data, baryon acoustic oscillations, and the Cosmic
Microwave Background measurements from Planck Space Observatory,
there are tentative indications that dark energy may not be perfectly
constant—as assumed in the standard Lambda-CDM Model—but could vary
slightly with cosmic time, possibly having been stronger in the past and
weakening today. These hints are not yet statistically decisive, but if
confirmed by upcoming surveys such as Euclid Space Telescope and
the Vera C Rubin Observatory, they would imply new physics beyond the
cosmological constant and significantly deepen our understanding of the
accelerated expansion of the universe.
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