I’d like to see an explanation for the weak gravitational interaction of dark matter. If this is the case, then comparatively speaking, isn’t the dark matter accelerating differently due to gravity when compared to known matter? I mean this in the curved space-time sense of acceleration; so a hypothetical “dark photon” wouldn’t travel in a strait line through curved space-time, but wold instead curve away from gravitational sources.

Or would dark matter exist in a parallel space-time overlapping ours where the curvature of space-time isn’t as severe? A model where different kinds of matter react to gravity differently would suggest this, I think. And I find that very hard to swallow.

…finally a testable hypothesis about dark matter.

Er…the Bullet Cluster…?

Nah, I think that “bullet cluster” one got shot down – despite being pretty quick on the draw!

@ 16 Gary Ansorge :

Awesome link there thanks!

Except it gets me wondering could Earth have rings like Saturn with a Moon like ours or would the Lunar gravity disturb and destroy the rings via tidal effects?

Stunning & well explained clip anyhow.

You’re missing the fact that humans are fallible biological organisms, and not hyperlogical robots. People are sloppy with language. That’s why we have Newton’s LAW of gravity, which is demonstrably false, and String “Theory,” which doesn’t really even have a way to be tested yet.

It isn’t like there is a strict hierarchy.

For illustration here & in historical order we have :

Kepler’s *laws* of gravity ->

-> explained by Newton’s *laws* of gravity ->

-> refined by Einstein’s theory of Relativity ->

-> confused by numerous theories of super strings and quantum physics and M-brane theory and WIMPs and Higg’s bosons and .. who knows what else they’ll think of next and we’re still really struggling to understand it all.

Seems to me that somewhere along the line shortly after Einstein, physics just went and totally lost the plot…

Or maybe its just me?

(I find cosmology confusing as all hell & my maths is pretty lousy. I am interested and read a lot about it – incl. here – but I still can’t seem to make much sense of it.)

According to the abstract the data suggest that we may see a haze of otherwise-inexplicable gamma rays around the galactic center, and that if it is not some kind of artifact its spherical shape seems to contraindicate an origin in baryonic matter. That last one seems a little silly to me given that the baryonic matter near the center of spiral galaxies usually forms a sphere, but I presume that’s because they’re pro astronomers who spell out the reason in the paper and I’m an abstract-reading fanboi.

…finally a testable hypothesis about dark matter.

Er…the Bullet Cluster…?

Torbjörn Larsson: I think I understand most of what you are saying, but, it does not appear to help very much outside a select group of people that “get it”.

Read Starts with a Bang. MOND actually complicates matters in that it would require gravity to be non-local. It may explain galaxy rotation curves, but that’s it. It doesn’t work for the bullet clusters.

Pete

Sorry, more people would remember if I had thought to turn the camera properly.

A very odd property about them is that they are self-annihilating: when two of them touch, they turn into energy

Are you sure they are WIMPs?!

But really, this isn’t so odd, photons are their own antiparticles as well, and the putative gravitons too I guess. Particles must have antiparticles for reasons of symmetry, and some interactions doesn’t seem to leave much other options than annihilating “self”.

This struck me as effectively downgrading gravity from a “law” to a “theory”.

Effectively a law is an isolated hypothesis, which predict and can be tested on one thing only.

A theory is by its nature a consistent set of related hypotheses, which can be tested on a multiplicity of predictions. Thus theories are the strongest knowledge we can have.

So todays cosmology that incorporates DM and DE is stronger than mere GR cosmologies. In fact, I believe I’ve seen claimed that they admit testing GR, “gravity law” if you dump the fancy “general relativity theory” (curved spacetime and so on) beneath if you will and just calculate, out to scales never before attempted.

For example, a detailed theory like evolution with its many subprocesses combining to the whole process of life, has made and tested predictions of every level of detail for 150 years now, published more or less daily as of today. To replace it we would need a theory that makes the same predictions but is simpler, or alternatively that makes at least one prediction more.

That is a very, very tall order.

A galactic halo peppered with innumerable brown dwarfs — which are probably MUCH more common in space than the few we’ve detected so far — would explain the Dark Matter phenomenon in a way that didn’t require any new and exotic kinds of matter to be invented.

But, sadly (for me), as time has worn on, the WIMPs seem to be winning.

In a nutshell a theory explains observations: Good ones explain a lot with a small set of variables, are free of internal contradictions and can be tested. A theory is not a description of reality: It is always temporary because the next observation can violate it.

I always had a bad feeling about the concepts of dark matter and even more dark energy: Observations had been made which proved that current theories were wrong, but instead of taking this a a rejection of these theories, a speculative new variable had been introduced. The concept of dark matter explains several things (gravitational lenses, rotation speed of galaxies, etc.), but with dark energy we are close to the border of real natural science (and I’m not sure at which side).

Popper introduced “falsifiability” as the criteria to distinguish a scientific theory from everything else. If a theory can not be proven wrong by observations, it is not science. I wonder if dark energy is a “testable” concept or only a gap filler in a model which did not stand the empirical test. Would be great to finally get some direct evidences which gives us the feeling to understand the world we live in a little bit better

Recommended readings: “Karl Popper”, “Epistemology”, “Falsifiability” and “Critical rationalism” on Wikipedia. Any recommendations from your side: Highly welcomed.

Good description. Another explanation is:

LAws are a mathematical description of the way the universe behaves/operates, as in,
F=MA
S=1/2 a*t^2
V=at
or F=G*M1*M2/R^2

or perhaps the most famous observation of all:

E=MC^2

These are all just mathematical descriptions of what we observe, thus they are LAWS. They in no way explain HOW all that happens, merely that it does. From these and other observations we construct a THEORY to explain why things act that way. As noted, all theories are approximations, but some are really GOOD approximations.

GAry 7

On a brighter note, for beauty, check out this link my Son left me on Facebook. It shows how a ring system might look if earth had one.

http://www.videosift.com/video/What-if-Earth-had-rings-like-Saturn

GAry 7

LAW is how things work
THEORY (please capitalize the T) is why they work

Remember, this is a ‘kindergarten’ explanation….

J/P=?