Too much gravity out there?
For many years now, astronomers have been able to calculate a reasonably good estimate of how much matter there is in the universe, based on information contained in the light and other radiation we can detect here on earth. They have also been able to calculate an alternative, independent estimate of how much matter exists, based on the observed gravitational effects of that matter. The two estimates are starkly different, the latter being much greater than the former. While some of the difference is undoubtly due to measurement error, the gap is far too large to be explained by this factor alone.
The leading hypothesis astronomers have come up with to explain the difference is that there may be large quantities of undetectable "dark matter" out there. This hypothetical matter is undetectable here on earth because the radiation it gives off is minimal, but its gravitational effects on the matter we can detect are quite evident. To explain the observed difference referred to above, dark matter would have to account for about 95% of all matter in the universe. The nature of this hypothetical dark matter is one of the central unresolved mysteries in 21st century cosmology.
For some years now though, Professor Milgrom of the Weizmann Institute in Israel has been propounding an alternative hypothesis: that Newton's second law of motion ("force is proportional to acceleration") does not hold when acceleration is very small. When acceleration is smaller than some threshold, force becomes proportional to the square of acceleration. He calls this Modified Newtonian Dynamics, or MOND. He has no theory as to why this might be, but it turns out that if MOND were true, the gravitational estimate of the quantity of matter in the universe would be much smaller and there would no longer be a need to hypothesize the existence of dark matter.
|
||||
 |
 |
 |
 |
 |
 |
 |
 |
 |
|
Compiled, developed and maintained by Philip Smith