The Quest for Cosmic Inflation

Cosmic Mysteries

Two startling discoveries during the late 20th century have rocked physics. The first is the discovery that ordinary matter, the stuff we are made of, is just a small part of our universe. In fact, a significant portion of the universe is made up of a substance called dark matter. The other is the discovery that most of the universe is made up of dark energy, a mysterious force that is responsible for the accelerating expansion of the universe. Dark energy and dark matter are cosmic mysteries that face cosmology today and frantic research is underway to unlock their true natures.

Dark Matter

Dark Matter is a cosmic mystery that faces cosmologists today. Dark matter cannot be seen or touched but is found to be widespread throughout the universe by making its presence felt by its gravitational pull on surrounding visible matter. Recent measurements show normal matter including all the visible stars planets and galaxies, makes up less than 5% of the total mass of the universe while dark matter is responsible for about 27% and dark energy for the rest 68%.

What dark matter is not

Scientists know what dark matter is not— much more than they know what it is

  • Galaxies or stars: Dark matter emits no light and cannot be viewed directly thus cannot be any visible astronomical object
  • Antimatter: Antimatter annihilates matter on contact producing gamma rays, but such annihilation signals have not been detected
  • Black holes: Black holes can bend light by acting as gravity lenses. Astronomers do not see enough lensing events to account for the amount of dark that must exist
  • Baryons: Baryons are normal matter particles. If dark matter were baryons it would have been detected via the light that is reflected.

That nonluminous matter could make up half of the mass of most galaxies was known to the astronomers for some time. If galaxies were made up only of visible matter, stars at a galaxy’s apparent periphery would revolve around the galactic center with a velocity that would decrease with its distance from the center. However, such a decrease was not observed. In the 1970s, American astronomers, Vera Rubin and Kent Ford, found that the outer stars were orbiting just as quickly as those at the center. The visible matter was not sufficient to account for this. However, Rubin and Ford were not the first to observe this phenomenon. In the 1930s Swiss-American astronomer Fritz Zwicky found that there was much more matter in the Coma cluster of galaxies than we can observe with a telescope (the Dutch astronomer Jan Oort showed the same for our local galactic neighborhood). Zwicky concluded that the Coma cluster must contain a large quantity of unseen matter that he called “dark matter” (from the German Dunkle Materie), with enough gravitational pull to keep the rapidly moving galaxies from flying apart.

Vera Rubin

Only two women have been honored with the Nobel Prize in physics, Marie Curie in 1903 and Maria Goeppert-Mayer in 1963. The dignified, unassuming, and brilliant American astronomer, Vera Rubin, is a woman pioneer who richly deserved the honor but which has eluded her so far. Rubin grew up at a time when very few women studied science. She earned a B.S. in astronomy from Vassar College in 1948 and was turned down by Princeton because Princeton did not accept women in its graduate astronomy program until 1975. Undaunted, she enrolled for her Master’s degree at Cornell University, where she studied physics under such luminaries as Philip Morrison, Richard Feynman, and Hans Bethe. Rubin conducted her doctoral work on the clustering of galaxies at Georgetown University under the supervision of George Gamow.

Rubin faced many obstacles in her career as an astronomer. Her first paper in 1950 on galactic clustering met with harsh reviews and when she and Kent Ford expanded that research in the 1970s, the result was no different. Rubin and Ford were forced to turn their focus into something different. They started to measure the rotational velocities of interstellar matter in orbit around the center of the nearby Andromeda galaxy. Their observations (confirmed by observations on other galaxies) led them to correctly conclude that the galaxies must contain significant amounts of dark matter.

Rubin is an active advocate for women’s equality in the sciences, especially astronomy. The Rubins have raised four children all of whom hold a Ph.D. in sciences, a feat that is no less admirable than her own scientific career.

The emergence of affordable mini-computers allowed physicists to perform numerical simulations to solve astronomical problems. In 1973, Princeton University astronomers Jeremiah Ostriker and James Peebles used N-body simulations to study how galaxies evolve. The scientists found in order to obtain the typical spiral or elliptical galactic structures that we see today in their simulation, they had to add a static, uniform distribution of mass 3 to 10 times the size of the total mass of the mass points. This seminal work was the first numerical evidence that dark matter was necessary to form the types of galaxies we observe in our universe today.

However, recent study of formation of galaxies has led cosmologists to believe that dark matter must really be in a different form from ordinary matter. One idea is that it could contain “supersymmetric particles” – hypothesized particles that are partners to those already known in the Standard Model of particle physics, or very light elementary particles such as axions or neutrinos. It may even consist of more exotic species of particles such as WIMPs (weakly interacting massive particles) that are predicted by modern theory of elementary particles but have not been discovered yet but experiments at the Large Hadron Collider (LHC) may provide more direct clues about dark matter.

Since 1978, Rubin and her team of astronomers have analyzed the spectra of over two hundred galaxies and found that nearly all contain significant amounts of dark matter. Her observed distribution of speeds of stars led to the conclusion that while the central parts of the spiral galaxies consists or ordinary stars, their peripheries are dominated by nonluminous or dark matter whose total mass is one or two times the mass of the visible galactic matter. Thus, Rubin’s work provided some of the first direct evidence for the existence of dark matter, verifying the earlier theoretical work of Jeremiah Ostriker and James Peebles.

So what exactly is dark matter? Scientists initially assumed dark matter is nothing but ordinary baryonic matter comprised of protons, neutrons, and electrons, but in some form that cannot be detected readily. This included gas clouds, or MACHOs—“massive compact halo objects” like white dwarfs or neutron stars, or even black holes.

However, recent study of formation of galaxies has led cosmologists to believe that dark matter must really be in a different form from ordinary matter. One idea is that it could contain “supersymmetric particles” – hypothesized particles that are partners to those already known in the Standard Model of particle physics, or very light elementary particles such as axions or neutrinos. It may even consist of more exotic species of particles such as WIMPs (weakly interacting massive particles) that are predicted by modern theory of elementary particles but have not been discovered yet but experiments at the Large Hadron Collider (LHC) may provide more direct clues about dark matter.

Dark Energy

All objects in the universe pull each other with a force we call gravity. The more the matter an object contains (its mass) the more gravity it has. This matter is made up of both visible matter, like stars, and invisible matter or dark matter.

It has been known since the late 1920s that the universe is expanding. The astronomers had determined the rate of the universe’s expansion. But since every bit of matter in the universe exerts a gravitational pull on every other bit, this should have created a hindrance causing the universe’s expansion to slow down even as it continued to expand from the energy of the Big Bang.

So the expectation was, the expansion must be slowing down and the astronomers wanted to measure the amount it had slowed down over the past billion years.

In the 1990s, two competing teams of astronomers—one led by Saul Perlmutter, at the Lawrence Berkeley National Laboratory, the other by Brian Schmidt, at Australian National University—set out to closely analyze a number of exploding stars, or supernovae 11, to gauge the universe’s progression. If the rate of expansion of the universe was uniform, the teams could calculate how bright the supernovae should appear at different points across the universe. The astronomers expected to determine how much the expansion of the universe was slowing down by comparing how much brighter the supernovae actually did appear. Surprisingly, when they looked 6 or 7 billion light-years away, they found that the supernovae were not brighter—and therefore nearer—than expected. They were dimmer, or more distant.

Both teams concluded that the expansion of the universe was not slowing down, rather speeding up.

In fact, there is only one way to explain the accelerating expansion of the universe. There must be a repulsive force embedded in the space itself that is capable of overcoming the inward pull of gravity. Just as gravity always pulls things together this new force must push them apart. Subsequent cosmological measurements have confirmed that roughly 68% of the universe by mass or energy consists of this anti-gravitational dark energy that is pushing the galaxies apart, though astronomers and physicists have no conclusive evidence of what it really is

The repulsive force is a property of space itself and could be a manifestation of the cosmological constant whose existence Einstein had postulated in 1917 to expand the fabric of space-time after his equations for general relativity would not allow for the cosmos to remain static, and then dropped by him when Edwin Hubble discovered that distant galaxies were flying away, suggesting that the universe was expanding.

Saul Perlmutter, Brian Schmidt, and Adam Riess shared the Nobel Prize in physics in 2011 for discovering that the universe is apparently expanding at an accelerated rate under the influence of a mysterious force that cosmologists now call dark energy, a finding that has revolutionized our current understanding of physics.


11 Supernovae are enormous stellar explosions marking the death of a star. A certain class of supernovae, called the Type Ia supernova, is particularly useful in measuring the distance to very distant galaxies. Type Ia supernovae, used by the astrophysicists in their studies, occur in two-star systems in which one star is at the end of its life cycle and is a white dwarf. This white dwarf may start pulling hydrogen off its partner star, increasing in its mass. If it reaches a mass 40% higher than the mass of the sun, a critical point is reached, and a massive nuclear explosion occurs.

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