Space Fan News #51: Dark Matter Galaxy; Most Distant Type 1a; Planck Completes Its Mission

http://gplus.to/TonyDarnell
http://facebook.com/SpaceFan

Dark Matter Galaxy:

http://web.mit.edu/physics/index.html
http://news.medill.northwestern.edu/chicago/news.aspx?id=198796
http://keckobservatory.org/news/most_distant_dwarf_galaxy_detected/

Farthest Type 1A SN:
http://hubblesite.org/newscenter/archive/releases/2012/02/text/

Planck Finishes Mission
http://www.esa.int/SPECIALS/Planck/SEMXWNMXDXG_0.html

Category:

Science & Technology

Tags:

• Space
• Space Fan News
• astronomy
• cosmology
• Space (TV Channel)

[firepower01]

So hard to not press the like button

[sbergman27]

@Baronstone "thats not dark matter, its a darn black hole."

The mass distribution is completely wrong for that. This is what? The 10th time I've had to point that out here? There is a substantial difference in the signature of that amount of mass distributed over the volume of a galaxy & that much mass concentrated at a point. 76 yrs of data & independent lines of reasoning forced scientists to recognize DM. But today we have armchair DM deniers who seem to have missed the history.

[HuskerSeaofRed]

I matter and I'm not dark at all.

[BenjaminSteber]

I think there is no dark matter. Video in my profile.

[1robon]

why the rush to conclude that this is proof of dark matter? couldnt it just be a distortion of spacetime due to large scale effects of the known forces. also looking back in time things might appear differently than expected.

[Baronstone]

Yeah a galaxy thats 120 million solar masses and invisible? Thats not dark matter, its a darn black hole.

[Battery9876]

Just to say that in the case of the orbit of a planet around the sun almost all the mass of the SS is in the sun so the theorem in fact applies.\

And I found it! watch?v=ERjkSbdn6-4 at 44:00. It's Newton's theorem.

Another thing I was thinking about is that if you use a stick to measure the galaxy, when space expands the stick expands too so does the galaxy get bigger measured with that stick?

Anyways sbergman27 I think we should conclude that we don't know.

[Battery9876]

I guess this theorem is valid for symetrical objects, like a disk of matter. However, as you say galaxies are not completely symetrical. Also gravitation travels at finite speed (light speed) apparently so it might have an effect on the validity of the theorem in that case.

Honestly I don't know, but I am not quite sure that you can infer the mass of the galaxy from just our sun.

For other galaxies sometimes they can use gravitational lensing to determine the mass but with our galaxy we can't.

[sbergman27]

@Battery9876 "which says that the gravitationnal effect of everything outside your orbit cancels out."

That works for idealized charged spherical shells. The matter outside Sol's orbit is not a spherical shell. It's a disk. When Earth's orbit about the sun takes it slightly closer to the galactic center than is the Sun, does the Sun's gravity suddenly get cancelled out and not matter any more? Of course not!

[Battery9876]

@sbergman27 I think there's a theorem - Gauss theorem? - check Susskind's cosmology lectures it's explained somewhere - which says that the gravitationnal effect of everything outside your orbit cancels out. If you use the speed of the sun around the center you will only find the mass within its orbit.

Also there may be a lot of DM outside the "luminous matter of the galaxy". According to the theorem you can't feel it. So you can't really know the mass of the galaxy and its escape velocity.

[Murphis55]

Keep up with your broadcasts I love hearing what you have to say. Nerds are the best! I am attempting to get going with amateur astronomy having purchased a Nexstar6. Great fun!

[sbergman27]

@Battery9876 Vₑ depends on starting position. ~500km/s is from Sol. 1000km/s is from the GC. I really should have used Vₑ from the edge, ~50,000ly out: ~400km/s. But the point is that local gravity swamps spatial expansion.

That said, we know from observation that Sol orbits the MW's mass center with a period (P) of ~200m yrs @ a distance (A) of ~1.7E9 AU. M=A³/P². So galactic mass is ~120E9 solar masses. Not sure why anyone would say we don't know the mass of our galaxy.

[Battery9876]

@sbergman27 From what I've read we don't know for sure the escape velocity of the galaxy. John Matters on that: watch?v=H6nP8m4QRUA. Also I have found another research paper from NASA which says 525km/s.

Also I think I would use the radius of the galaxy, not the diameter, so that would be 1km/sec, not 2.

In any case, ve=sqrt(2GMR/r) so r=2GMR/ve^2. Using your numbers, a change by a factor of 1.002 of ve would translate to a change of 0.4% of r.

In the early universe it might have been different.

[TheisticThinker]

@JamezGotSwag The sad thing is that in nowadays, someone who actually cares about something greater than him/herself is called a 'nerd'. You are 'normal' if you just doesn't care or isn't interested in Science etc. It's kinda sick. :/

[Nekobe24]

"It has gravity and that gravity bends light that travels through any dark matter that might be present. When it does that we have it. It is betrayed." -- I love that line XD That and the TNG reference are awesome.

[sbergman27]

@Battery9876 Grab a pencil & napkin & let's check. :-) The expansion rate, known as the Hubble constant, is around 70 km/sec/megaparsec. Our galaxy is ~30000 parsecs across, or ~0.03 megaparsecs. 70km/s/mPa * 0.03 mPa = ~2 km/s.

For comparison, the escape velocity of our galaxy is ~1000km/s. Gravity is acting to offset the 2km/s expansion between opposite ends of our galaxy. Based on that how much larger would you guesstimate our galaxy is due to spatial expansion? Is it expanding?

[Battery9876]

@sbergman27 To tell you the truth I listened to Carroll's explanation many times and I am still not to sure I understand.

Btw I made a mistake in my previous posts,the stars at the periphery of galaxies move too fast not too slow that's why they need dark matter!!

If the expansion really has an effect,this makes me wonder if galaxies wouldn't change in size when the rate of expansion of space changes.The effect of the expansion is very small at our scale but it might be large for a galaxy.mmh.

[sbergman27]

@Battery9876 I've never been quite sure what Carroll was saying there. The issue is highly confused by his inclusion of a black hole, a region of space cut off by its schwarchild radius. The Earth's orbital distance *is* very slightly larger than it would be without expansion. But immeasurably so. In fact, the space in your living room is expanding by ~10% per billion yrs. But the EM bonds of the matter in the room hold the distance between the sofa and coffee table (almost) fixed.

[JamezGotSwag]

Feel like a nerd watching this but who gives AF, It's so interesting!!! You sir have a new sub:)

[Battery9876]

@Battery9876 This being said, I know that astronomers have probably good reasons to think that dark matter exists. I probably misunderstood something somewhere.

[Battery9876]

@Battery9876 The magnitude of the effect of the expansion of space would be roughly proportional to the distance from the center of the galaxy. This could explain the discrepancy with Kepler's laws as we get further from the center.

Also, if this is correct, it could explain why the first galaxies were small. They were small because the expansion of space was very fast, and everything was spreading apart. Later the expansion slowed and large galaxies could form by coalescence of smaller ones.

[Battery9876]

@Battery9876 However, from what understand, space is a medium that can be curved. The theory of general relativity says that matter can curve it. But I imagine that if new space is created it will also change its shape.

If that's correct, the stars at the periphery of galaxies will feel the expansion inside the galaxy and it  will have the effect counter gravity to some extent . The result is that gravity will be weaker and that a lower speed will be required to stay in orbit around the center.

[Battery9876]

@sbergman27 Well, I am still a bit confused. From what I understand there are 2 lines of thinking here: one , which is what you say and what is described in the video "The Expanding Universe" which is that indeed gravity is strong enough to compensate. The other is that the expansion of space is completely transparent, gravity is a force in 1 over d squared and that's it, that's what Sean Carroll says in the video watch?v=vUNtO2r_-eo at 26:00.

[lucabaracuda987]

@tdarnell hey T just had a bunch load of beers and a bottle of Gin cause its Australia Day and wanted to drop in to give you a friendly /hug. So don't ask me why I'm doing it. I just feel like it :)