The Basic Issue
Extragalactic systems (like the spiral galaxy NGC 7757 pictured at right)
show clear evidence for mass discrepancies. When we measure the amount
of luminous mass (stars and gas) in these systems and apply Newton's
Law of Gravity, the observed mass falls well short
of the amount required to explain the observed orbital speeds.
This is the mass discrepancy.
There are two logical possibilities to explain the observed mass discrepancies:
- Most of the Mass in the Universe is Invisible (Dark Matter), or
- Dynamical Laws must be Modified (MOND).
There are significant challenges to both ideas.
Issues for MOND:
- Can a modified force law explain all observations?
- Can a satisfactory theory encompassing both General Relativity and
MOND be found?
Issues for Dark Matter:
- Does the stuff we call Dark Matter really exist?
- Can a dark matter based theory explain the MONDian phenomenology observed
in rotation curves?
Why should the mass discrepancy only appear at a particular acceleration scale?
My gut reaction to all these questions is negative.
But it appears that one set or the other must be answered in the affirmative.
Either way, we are missing something fundamental about the nature of our