How modified gravity works (a FAQ)

M

ModifiedGravityNerd

Registered Member
Hi everyone! I'm sure you've heard that most the mass in the universe is mysterious dark matter. But is that really the case? We sure do see more gravity than can be explained by the ordinary matter in stars and gas using gravity as we know it but perhaps we are simply wrong about how gravity works? It is precisely this idea that Mordehai Milgrom in 1983 decided to persue. He created what is now know as modified Newtonian dynamics (MOND). This is the leading modified gravity theory and has had a large number of predictions come true. Because most people don't know much about this theory I've created a website that answers the most frequently asked questions about it. It discusses what it can do (rotation curves), what it sort of does (lensing) and why it so far fails to replace general relativity (clusters, structure formation, CMB and BBN). Hopefully some of you find it a useful reference :)

MOND frequently asked questions

If you have any further questions I'd be happy to answer them!
 
ModifiedGravityNerd said:
Hi everyone! I'm sure you've heard that most the mass in the universe is mysterious dark matter. But is that really the case? We sure do see more gravity than can be explained by the ordinary matter in stars and gas using gravity as we know it but perhaps we are simply wrong about how gravity works? It is precisely this idea that Mordehai Milgrom in 1983 decided to persue. He created what is now know as modified Newtonian dynamics (MOND). This is the leading modified gravity theory and has had a large number of predictions come true. Because most people don't know much about this theory I've created a website that answers the most frequently asked questions about it. It discusses what it can do (rotation curves), what it sort of does (lensing) and why it so far fails to replace general relativity (clusters, structure formation, CMB and BBN). Hopefully some of you find it a useful reference :)

MOND frequently asked questions

If you have any further questions I'd be happy to answer them!
Click to expand...
For those of us who can read Wiki, what’s the advantage of your website?
 
OK, but it's still Newtonian, right? It's not compatible with 20th/21st century Einsteinian relativity, correct? For example, it struggles to explain gravitational lensing, yes?
 
exchemist said:
For those of us who can read Wiki, what’s the advantage of your website?
Click to expand...
The Wikipedia article (which I wrote too basically) is a good but short introduction. It contains far fewer images and graphs due to the strict copyright rules. It also doesn't address common misunderstandings because Wikipedia is an encyclopedia not a textbook. You'd have to first show a secondary source that claims a misunderstanding is common and then another source to disprove it. That just doesn't exist for topics like this. The FAQ is designed to get someone familiar enough with MOND to understand the primary literature and contribute to the field.
 
DaveC426913 said:
OK, but it's still Newtonian, right? It's not compatible with 20th/21st century Einsteinian relativity, correct? For example, it struggles to explain gravitational lensing, yes?
Click to expand...
That's question 23 of the FAQ

In short, yes it is not relativistic so it can't do lensing or cosmology. However, if you use GR and calculate the amount of dark matter needed it is exactly how much extra gravity you'd expect from MOND. So Milgrom's Law also seems to apply to relativistic effects.
 
ModifiedGravityNerd said:
The Wikipedia article (which I wrote too basically) is a good but short introduction. It contains far fewer images and graphs due to the strict copyright rules. It also doesn't address common misunderstandings because Wikipedia is an encyclopedia not a textbook. You'd have to first show a secondary source that claims a misunderstanding is common and then another source to disprove it. That just doesn't exist for topics like this. The FAQ is designed to get someone familiar enough with MOND to understand the primary literature and contribute to the field.
Click to expand...
OK that’s clear enough. You are from the Netherlands I see.
 
ModifiedGravityNerd said:
That's question 23 of the FAQ

In short, yes it is not relativistic so it can't do lensing or cosmology. However, if you use GR and calculate the amount of dark matter needed it is exactly how much extra gravity you'd expect from MOND. So Milgrom's Law also seems to apply to relativistic effects.
Click to expand...
But doesn't GR already completely explain gravitational lensing? So if MOND added to that, shouldn't expect twice as much lending as we see, or am I misunderstanding?
 
Mond mirrors one aspect of GR. Not enough for replacement.

Kalusa/Klein united GR and EM, but had an unidentified scalar field so it was rejected.
 
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