April 9, 2008
The Dark Side of the Universe
Neta A. Bahcall
Professor of Astrophysical Sciences, Princeton University
Minutes of the 27th Meeting of the 66th Year
President Joe Giordmaine convened the meeting at 10:15 AM. This being the 27th meeting of the 66th year since the founding the the Old Guard. George Hansen let the invocation.
Bill Schoelwer then gave the minutes of the meeting of April 2 during which
Evan Thomas, Ferris Professor of Journalism of Princeton University discussed “The 2008 Presidential Campaign.” Harold Borkin introduced his visitors, Charlotte Zanidakis and Hazel Stix.
Harold Borkin also introduced Neta A. Bahcall, Eugene Higgins Professor of Astrophysics, Princeton University whose presentation was entitled “The Dark Side of the Universe.” Ms Bahcall of the Department of Astrophysical Sciences lists as her fields of interest: Observational cosmology, dark matter and the mass density of the universe, the formation and evolution of clusters of galaxies and the quasars and their environment. Ms. Bahcall is Director of the Undergraduate Program and Director of the Council on Science and Technology. She has had an illustrious 30 year career. I find she has 287 published papers and has received many honors. She chairs the Time Allocation Committee of NASA for the Hubble Space Telescope. (Parenthetically, Neta’s late husband, John Bahcall supported vigorously the development of the Hubble telescope that was proposed and nurtured to reality by Lyman Spitzer of Princeton University, whom many of you knew, and George Field. At the time, I was at the then Perkin Elmer Corporation, designers and and builders of the telescope. Early observation results were disastrous because of a focusing error that was later corrected after another costly space flight for the purpose of replacing parts to take care of the problem.
Ms. Bahcall introduced her talk by emphasizing that she would be reviewing new and unexpected information about the universe, stemming from observations made only in the last decades. The new model that has emerged is the Lambda-CDM ,or concordance model, which holds that 22 percent of the universe is dark matter that pulls the universe together and 74 percent is dark energy which pushes the universe apart. The rest is atomic matter. The universe is thought to be expanding an an accelerating rate. The new theory of the universe appears, through observation, to be quite successful and is now the starting point for new cosmological observations and conclusions. Neta Bahcall was an important force in developing the new theory.
Neta Bahcall was an early advocate of the idea that the universe was light, that is, lacked enough mass to explain observations of the large scale structure. The distribution of galaxies stretching back in time, suggested that the ordinary atoms and cold dark matter accounted for only a third of the mass of what was required. There must be something more that we were unaware of.
That was contrary to the existing model of the big bang which called for a flat universe with a critical density of one: just enough energy and matter to keep the universe expanding forever without falling back on itself.
She explained aspects of the new theory in her talk.
What we see of the universe are shining stars and galaxies, the visible radiation that originates in nuclear reactions. Not seen is radiation from the other parts of the electromagnetic spectrum, which can be measured with instruments. But, not seen by eye or instruments, is the presence of dark matter, inferred from the observation of the tangential velocity of stars and dark halos rotating about the center of the galaxies. Stars in galaxies have a velocity of about 200 km/sec. The mass of this dark matter is inferred from the lensing effect as light streams past the dark halos surrounding all galaxies and is concentrated by gravity acting on light photons.
Dark matter or gravity, is an unknown entity. But the amount of dark matter can be measured. It does not appear to be atomic matter. It may be made of new exotic particles. There are theories about the nature of the particles but no evidence Dark matter appears to exist everywhere. The particles of dark matter may act like neutrinos in that they may pass unobstructed through ordinary matter. The individual particles may have near zero mass but their numbers account for about 22 percent of the total mass of the universe. Dark matter and atomic matter pulls the universe together through gravity, but do not have enough density to present a steady expansion of the universe.
More about dark matter: observations of the Andromeda galaxy which is like our Milky Way galaxy, have revealed interesting information. Measurements in the rotating arms of the galaxy reveal a constant velocity of stars around the galaxy center. But the same velocity is measured in the dark halo region where one would not expect such a measurement. This region is thought to be composed of dark matter and comprises a large part of all galaxies and about 20 percent of the mass of the universe. The Hadron Collider Accelerator will be used to conduct experiments to look for unknown particles possibly making up dark matter.
About 70 years ago Edwin Hubble confirmed that the universe was expanding at a constant rate. Einstein, in an attempt to postulate a stable universe, introduced a cosmological constant, a fudge factor, in his equations to bring them in conformance with the observations. But recently, light measurements from distant supernova indicate that the expansion of the universe is actually accelerating. So it is necessary to introduce a new fudge factor, Dark Energy, or anti-gravity.
Dark Energy, thought to account for 74 percent of the universe, is postulated to explain the speed up in the expansion of the universe This force acts in opposing the gravity exerted by the distributed mass of the universe. Because it is larger, the universe not only expands but does so at an accelerating rate.
As stated, dark energy can be inferred by the velocity of the dark halos of galaxies, the mass can be determined through the lens effect as light streams past the halos and is concentrated through the gravity force of the galaxy.
So, dark energy and dark matter are not observed directly. They have been introduced as constructs to explain what is observed.
Whether the universe will continue to expand depends on amount of dark matter and energy.
If these turn out to variable forces, the universe may expand and rip apart or contract and collapse on itself. Astrophysicists are most interested in developing more information about these entities.
Ms Bahcall used a Power Point presentation to make her points and repeated the main themes of her talk in different ways.
Although the new theory of the universe appears to accord with observations, it must be frustrating to attribute what is being observed to dark energy and matter about which nothing is really known.
The critical density of the universe is 6 photons/cubic meter. This is the density beyond which the universe would start to slow in expansion.
She showed a large scale map of the universe called the Great Wall.
Ms. Bahcall invited questions during and after the talk:
About life elsewhere in the universe– There are so many stars and planets that there is a probability that the conditions are similar to ours and therefor should support some form of life.
About lensing effect– light ( photons) have mass As they pass celestial bodies the light rays are bent inward by the gravity force of the body. The body therefor acts as a lens in concentrating the light rays.
What about the theory of the Big Bounce as opposed to the Big Bank. - The big bounce holds that the universe cycles , ceasing to expand and then collapsing on itself repeatedly. Each time with slightly different states, depending on quantum effects. Observations to date do not support the Big Bounce theory..
Does the sun cause fluctuation in the earth’s temperature.-- could be
The Big Bang - conditions prior to the event, time and space are not known; The string theory may sometime contain the explanation.
Black holes - there are many with gravity strong enough to draw in light.
She concluded by saying that the Universe will grow larger, sparser, darker and colder.
In doing the minutes, I ran across the following recipe for the universe on the Internet:
“Take one part unidentified goop. Add three parts mysterious energy. Throw in a dash of ordinary atoms, mix, compress, explode. Let expand for 13.7 billion years.”
Neta Bahcall is in complete command of her subject and dealt with a highly complex area of physics in a clear and interesting presentation.
The meeting adjoined at 11:40 AM.
Respectfully submitted,
Ernest W. Schlieben
Bill Schoelwer then gave the minutes of the meeting of April 2 during which
Evan Thomas, Ferris Professor of Journalism of Princeton University discussed “The 2008 Presidential Campaign.” Harold Borkin introduced his visitors, Charlotte Zanidakis and Hazel Stix.
Harold Borkin also introduced Neta A. Bahcall, Eugene Higgins Professor of Astrophysics, Princeton University whose presentation was entitled “The Dark Side of the Universe.” Ms Bahcall of the Department of Astrophysical Sciences lists as her fields of interest: Observational cosmology, dark matter and the mass density of the universe, the formation and evolution of clusters of galaxies and the quasars and their environment. Ms. Bahcall is Director of the Undergraduate Program and Director of the Council on Science and Technology. She has had an illustrious 30 year career. I find she has 287 published papers and has received many honors. She chairs the Time Allocation Committee of NASA for the Hubble Space Telescope. (Parenthetically, Neta’s late husband, John Bahcall supported vigorously the development of the Hubble telescope that was proposed and nurtured to reality by Lyman Spitzer of Princeton University, whom many of you knew, and George Field. At the time, I was at the then Perkin Elmer Corporation, designers and and builders of the telescope. Early observation results were disastrous because of a focusing error that was later corrected after another costly space flight for the purpose of replacing parts to take care of the problem.
Ms. Bahcall introduced her talk by emphasizing that she would be reviewing new and unexpected information about the universe, stemming from observations made only in the last decades. The new model that has emerged is the Lambda-CDM ,or concordance model, which holds that 22 percent of the universe is dark matter that pulls the universe together and 74 percent is dark energy which pushes the universe apart. The rest is atomic matter. The universe is thought to be expanding an an accelerating rate. The new theory of the universe appears, through observation, to be quite successful and is now the starting point for new cosmological observations and conclusions. Neta Bahcall was an important force in developing the new theory.
Neta Bahcall was an early advocate of the idea that the universe was light, that is, lacked enough mass to explain observations of the large scale structure. The distribution of galaxies stretching back in time, suggested that the ordinary atoms and cold dark matter accounted for only a third of the mass of what was required. There must be something more that we were unaware of.
That was contrary to the existing model of the big bang which called for a flat universe with a critical density of one: just enough energy and matter to keep the universe expanding forever without falling back on itself.
She explained aspects of the new theory in her talk.
What we see of the universe are shining stars and galaxies, the visible radiation that originates in nuclear reactions. Not seen is radiation from the other parts of the electromagnetic spectrum, which can be measured with instruments. But, not seen by eye or instruments, is the presence of dark matter, inferred from the observation of the tangential velocity of stars and dark halos rotating about the center of the galaxies. Stars in galaxies have a velocity of about 200 km/sec. The mass of this dark matter is inferred from the lensing effect as light streams past the dark halos surrounding all galaxies and is concentrated by gravity acting on light photons.
Dark matter or gravity, is an unknown entity. But the amount of dark matter can be measured. It does not appear to be atomic matter. It may be made of new exotic particles. There are theories about the nature of the particles but no evidence Dark matter appears to exist everywhere. The particles of dark matter may act like neutrinos in that they may pass unobstructed through ordinary matter. The individual particles may have near zero mass but their numbers account for about 22 percent of the total mass of the universe. Dark matter and atomic matter pulls the universe together through gravity, but do not have enough density to present a steady expansion of the universe.
More about dark matter: observations of the Andromeda galaxy which is like our Milky Way galaxy, have revealed interesting information. Measurements in the rotating arms of the galaxy reveal a constant velocity of stars around the galaxy center. But the same velocity is measured in the dark halo region where one would not expect such a measurement. This region is thought to be composed of dark matter and comprises a large part of all galaxies and about 20 percent of the mass of the universe. The Hadron Collider Accelerator will be used to conduct experiments to look for unknown particles possibly making up dark matter.
About 70 years ago Edwin Hubble confirmed that the universe was expanding at a constant rate. Einstein, in an attempt to postulate a stable universe, introduced a cosmological constant, a fudge factor, in his equations to bring them in conformance with the observations. But recently, light measurements from distant supernova indicate that the expansion of the universe is actually accelerating. So it is necessary to introduce a new fudge factor, Dark Energy, or anti-gravity.
Dark Energy, thought to account for 74 percent of the universe, is postulated to explain the speed up in the expansion of the universe This force acts in opposing the gravity exerted by the distributed mass of the universe. Because it is larger, the universe not only expands but does so at an accelerating rate.
As stated, dark energy can be inferred by the velocity of the dark halos of galaxies, the mass can be determined through the lens effect as light streams past the halos and is concentrated through the gravity force of the galaxy.
So, dark energy and dark matter are not observed directly. They have been introduced as constructs to explain what is observed.
Whether the universe will continue to expand depends on amount of dark matter and energy.
If these turn out to variable forces, the universe may expand and rip apart or contract and collapse on itself. Astrophysicists are most interested in developing more information about these entities.
Ms Bahcall used a Power Point presentation to make her points and repeated the main themes of her talk in different ways.
Although the new theory of the universe appears to accord with observations, it must be frustrating to attribute what is being observed to dark energy and matter about which nothing is really known.
The critical density of the universe is 6 photons/cubic meter. This is the density beyond which the universe would start to slow in expansion.
She showed a large scale map of the universe called the Great Wall.
Ms. Bahcall invited questions during and after the talk:
About life elsewhere in the universe– There are so many stars and planets that there is a probability that the conditions are similar to ours and therefor should support some form of life.
About lensing effect– light ( photons) have mass As they pass celestial bodies the light rays are bent inward by the gravity force of the body. The body therefor acts as a lens in concentrating the light rays.
What about the theory of the Big Bounce as opposed to the Big Bank. - The big bounce holds that the universe cycles , ceasing to expand and then collapsing on itself repeatedly. Each time with slightly different states, depending on quantum effects. Observations to date do not support the Big Bounce theory..
Does the sun cause fluctuation in the earth’s temperature.-- could be
The Big Bang - conditions prior to the event, time and space are not known; The string theory may sometime contain the explanation.
Black holes - there are many with gravity strong enough to draw in light.
She concluded by saying that the Universe will grow larger, sparser, darker and colder.
In doing the minutes, I ran across the following recipe for the universe on the Internet:
“Take one part unidentified goop. Add three parts mysterious energy. Throw in a dash of ordinary atoms, mix, compress, explode. Let expand for 13.7 billion years.”
Neta Bahcall is in complete command of her subject and dealt with a highly complex area of physics in a clear and interesting presentation.
The meeting adjoined at 11:40 AM.
Respectfully submitted,
Ernest W. Schlieben