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R. N. Mohapatra
with Nobel Prize winning physicist Robert Millikan. He spent his professional
life at the California Institute of Technology. He had an eccentric personality
and many people thought of him as a curmudgeon who used the choicest
words for people he did not like. He had many accomplishments in astronomy.
For example, he proposed the existence of neutron stars, as already noted.
He also proposed that galaxies can form clusters, but his most profound
proposal was that there may be dark matter in the galaxies. He analyzed the
redshifts of galaxies inside the Coma Cluster and found that their velocities
were much larger than what would be expected if gravitational attraction of
only visible matter in the cluster was the cause. He needed to have more matter
to understand the observed speeds. He ascribed this discrepancy between
observed matter motion (familiar matter) and the one inferred using the
gravitational effect of visible matter to be due to something he called “Dunkel
materi” (or dark matter). This was in 1933, and those were the early days of
cosmology. So this did not become a subject of great importance then as it is
now.
The next big step came with the thorough and meticulous research by
Vera Rubin who studied the rotation curves of hundreds of spiral galaxies.
A rotation curve is the speed of stars and gas at different distances from the
center of the galaxy, measured using the redshift of light from them.
1 She
found that the velocities of stars or gas farther out from the center of the galaxy
did not go down as expected on the basis of gravitational force. For example,
this is clearly established for planets. But speed remained constant even as the
distance was further (Fig. 24.1). One way to understand this is to assume that
there is dark matter in every galaxy. This confirmed Zwicky’s suspicion that
there was indeed hidden matter in galaxies which did not emit or reflect light,
but was still there, exerting gravitational force on objects farther away from
the center of the galaxy so as to make their speeds remain constant instead of
going down.
The question is how does one understand the constancy of the rotation
speeds observed (Fig. 24.1)? The existence of dark matter is one way to
understand it. But another way is to postulate that laws of gravity change
at large distances. The latter theory is called Modified Newtonian Dynamics
1 Redshift is the analogous phenomenon to Doppler shift of sound which you experience every day. When
you are standing still on a road side and a police car with siren on passes by you, first as the policeman
approaches you, the sound frequency is higher (more shrill) and when the car is going away, the sound
frequency is lower. This is a phenomenon which happens with all waves and since light is a wave, it
happens to it too. When a star is moving towards Earth, its light frequency will increase. The frequency
will be shifted towards blue (called blue shift) and when it is moving away from you, the frequency goes
the opposite way i.e. the light will be redshifted.
R. N. Mohapatra
with Nobel Prize winning physicist Robert Millikan. He spent his professional
life at the California Institute of Technology. He had an eccentric personality
and many people thought of him as a curmudgeon who used the choicest
words for people he did not like. He had many accomplishments in astronomy.
For example, he proposed the existence of neutron stars, as already noted.
He also proposed that galaxies can form clusters, but his most profound
proposal was that there may be dark matter in the galaxies. He analyzed the
redshifts of galaxies inside the Coma Cluster and found that their velocities
were much larger than what would be expected if gravitational attraction of
only visible matter in the cluster was the cause. He needed to have more matter
to understand the observed speeds. He ascribed this discrepancy between
observed matter motion (familiar matter) and the one inferred using the
gravitational effect of visible matter to be due to something he called “Dunkel
materi” (or dark matter). This was in 1933, and those were the early days of
cosmology. So this did not become a subject of great importance then as it is
now.
The next big step came with the thorough and meticulous research by
Vera Rubin who studied the rotation curves of hundreds of spiral galaxies.
A rotation curve is the speed of stars and gas at different distances from the
center of the galaxy, measured using the redshift of light from them.
1 She
found that the velocities of stars or gas farther out from the center of the galaxy
did not go down as expected on the basis of gravitational force. For example,
this is clearly established for planets. But speed remained constant even as the
distance was further (Fig. 24.1). One way to understand this is to assume that
there is dark matter in every galaxy. This confirmed Zwicky’s suspicion that
there was indeed hidden matter in galaxies which did not emit or reflect light,
but was still there, exerting gravitational force on objects farther away from
the center of the galaxy so as to make their speeds remain constant instead of
going down.
The question is how does one understand the constancy of the rotation
speeds observed (Fig. 24.1)? The existence of dark matter is one way to
understand it. But another way is to postulate that laws of gravity change
at large distances. The latter theory is called Modified Newtonian Dynamics
1 Redshift is the analogous phenomenon to Doppler shift of sound which you experience every day. When
you are standing still on a road side and a police car with siren on passes by you, first as the policeman
approaches you, the sound frequency is higher (more shrill) and when the car is going away, the sound
frequency is lower. This is a phenomenon which happens with all waves and since light is a wave, it
happens to it too. When a star is moving towards Earth, its light frequency will increase. The frequency
will be shifted towards blue (called blue shift) and when it is moving away from you, the frequency goes
the opposite way i.e. the light will be redshifted.