Starship Asterisk*

by **Nitpicker** » Thu Oct 10, 2013 12:48 am

stephen63 wrote:
Nitpicker wrote: But one can still ponder the interaction between two distant galaxies, as in this APOD, with only a 19th century understanding. It still makes sense qualitatively.
To a non professional astronomer, observations, for the most part, are qualitative!

For galaxies and lots of other deep sky thingies, I think you are right. But there seem to be an increasing number of amateurs doing useful, quantitative things like, for example: astrometric measurements of binaries, accurate timings of lunar occultations, mapping meteor showers, supernova hunting, and even measuring the brightness and spectrum of variable stars. And some others, including me, enjoy doing less useful things, like measuring (as best I can from my rudimentary photos) the motion of already well-known Solar System objects. I find it calming to know that I haven't yet observed any discrepancies in the motion of the Moon and planets predicted by the professionals. It also compensates for the fact that I'm unlikely to ever develop the talent for the beautiful deep sky photography so often seen on this site. We all have our quirks.

by **Chris Peterson** » Wed Oct 09, 2013 9:00 pm

Anthony Barreiro wrote:
Chris Peterson wrote:I would suggest this has little to do with our increased understanding of the Universe, but rather represents a shift in our approach to scientific understanding in general. It is a viewpoint that extends to all areas of science, not just astronomy.
I thought that's what I said. The last time I checked, everything was part of the universe. Thanks for clarifying. :lol2:

I wondered. It isn't always clear what people mean when they talk about the Universe. As a physicist, I tend to view everything as a subset of physics: astronomy, cosmology, biology, chemistry. But some non-physicists don't see it that way.

by **Anthony Barreiro** » Wed Oct 09, 2013 8:54 pm

Chris Peterson wrote:
Anthony Barreiro wrote:It's interesting to me that as we learn more about the history of the universe, fewer and fewer things appear static, and more and more things are revealed as processes. So an Arp galaxy is only peculiar because we happen to see it at a moment of intense interaction and change.
I would suggest this has little to do with our increased understanding of the Universe, but rather represents a shift in our approach to scientific understanding in general. It is a viewpoint that extends to all areas of science, not just astronomy.

I thought that's what I said. The last time I checked, everything was part of the universe. Thanks for clarifying.

by **stephen63** » Wed Oct 09, 2013 7:51 pm

Nitpicker wrote: But one can still ponder the interaction between two distant galaxies, as in this APOD, with only a 19th century understanding. It still makes sense qualitatively.

To a non professional astronomer, observations, for the most part, are qualitative!

by **Chris Peterson** » Wed Oct 09, 2013 7:50 pm

Anthony Barreiro wrote:It's interesting to me that as we learn more about the history of the universe, fewer and fewer things appear static, and more and more things are revealed as processes. So an Arp galaxy is only peculiar because we happen to see it at a moment of intense interaction and change.

I would suggest this has little to do with our increased understanding of the Universe, but rather represents a shift in our approach to scientific understanding in general. It is a viewpoint that extends to all areas of science, not just astronomy.

by **Anthony Barreiro** » Wed Oct 09, 2013 7:31 pm

APOD Robot wrote: ... Over 50 million light-years distant toward the constellation Leo, the pair's appearance has earned them the designation Arp 94 in the classic catalog of peculiar galaxies. But such galactic collisions and mergers are now thought to represent a normal course in the evolution of galaxies, including our own Milky Way. ...

It's interesting to me that as we learn more about the history of the universe, fewer and fewer things appear static, and more and more things are revealed as processes. So an Arp galaxy is only peculiar because we happen to see it at a moment of intense interaction and change.

By the way, this is a lovely picture, and I would name this the "Sea Turtle Galaxy Pair."

by **Nitpicker** » Wed Oct 09, 2013 4:56 pm

Chris Peterson wrote:So you've never looked at a galaxy through your telescope? Because the actual structure of galaxies is as dependent upon the existence of dark matter as on gravity. Without dark matter, galaxies would look very different (if they existed at all). So your lack of a need to infer the existence of dark matter is merely a consequence of your degree of knowledge, not of what you were actually observing. (Please don't take that as an insult... it certainly isn't meant that way. It is only an observation that our knowledge fundamentally colors how we interpret our observations.)

Not insulted at all Chris. If I hadn't read about the existence of dark matter, I would have no way to tell that the few barely visible galaxies I can see in my modest scope, do not have enough (conventional) matter to explain their structure and (unobservable) motion. Not sure I'd even know how to tell with a professional scope.

My point was only that if a bona-fide layperson wants to get a better understanding of the universe, they might start with the Big Bang and gravity. If they are still paying attention, they might then consider dark matter. And if they really want to ponder the ultimate fate of the universe, they might then consider dark energy.

But one can still ponder the interaction between two distant galaxies, as in this APOD, with only a 19th century understanding. It still makes sense qualitatively.

by **Chris Peterson** » Wed Oct 09, 2013 4:24 pm

Nitpicker wrote:Personally, as a rank amateur with a six-inch telescope, I have never made an observation which has required me to infer the existence of dark matter, nor dark energy. I am, however, quite comfortable with gravity, and I find it has a great many uses in my day to day life as well. Call me simple if you must.

So you've never looked at a galaxy through your telescope? Because the actual structure of galaxies is as dependent upon the existence of dark matter as on gravity. Without dark matter, galaxies would look very different (if they existed at all). So your lack of a need to infer the existence of dark matter is merely a consequence of your degree of knowledge, not of what you were actually observing. (Please don't take that as an insult... it certainly isn't meant that way. It is only an observation that our knowledge fundamentally colors how we interpret our observations.)

by **Nitpicker** » Wed Oct 09, 2013 4:11 pm

BDanielMayfield wrote:What’s pulling the universe apart? This is one of the biggest mysteries of astrophysics.I could state what I strongly believe to be the cause, but doing so would be “frowned upon, in this establishment.”

For the record, I accept that dark energy -- whatever it is -- creates the force that is accelerating the expansion of the universe, mainly because:

1) It has a robust mathematical form, however inferred.

2) I can't think of a more reasonable explanation.

3) It's not likely to affect anyone I care about in the near future.

by **Chris Peterson** » Wed Oct 09, 2013 4:09 pm

BDanielMayfield wrote:Then the modern Big Bang theory, as modified to account for accelerating universal expansion, is a theory that explains observations, but it does so with another major, fundamental unknown plugged in, which is dark energy. As you stated yourself Chris, dark energy’s “actual nature remains very unclear.”

I fear that even the scientifically literate tend to misunderstand what science is all about, sometimes.

Every theory is simply a mechanism designed to explain observations. Most non-trivial theories have undergone revisions with time as increasingly rich observations revealed previously unrecognized details.

Dark energy isn't a fundamental unknown. It is an observation, and by adjusting our cosmological model to account for it we are able to make other sorts of predictions that we couldn't before, and about more than simply the observation of increased universal expansion. That's one thing that results in increased confidence that our model is improving.

What does it even mean that some fundamental thing like dark energy is "unknown", or "unclear"? What isn't? We know nothing about what electric charge "really" is. We know nothing about what gravity "really" is. We know nothing about what any of the fundamental forces "really" are. Each of these things is just like dark energy: mysterious "forces" (even that word is applied as a simplification) which we can only describe or explain by using abstract theories that appear to describe the behavior we observe.

Fundamentally, dark energy is no more mysterious than electric charge or gravity. The only difference is that we don't yet have as richly developed a theory for it, we don't yet have as many independent observations of its effects.

by **FloridaMike** » Wed Oct 09, 2013 4:01 pm

Chris Peterson wrote: ..... the rate of expansion of the Universe is actually increasing- something that can only be explained classically by a force opposite that of gravity. We call that force "dark energy", although its actual nature remains very unclear.

Kudos Chris,
THIS is how to describe the speculations science makes so that they do not get confused with more settled matters.

by **BDanielMayfield** » Wed Oct 09, 2013 3:49 pm

Nitpicker wrote:The Big Bang presumably created matter, dark matter and dark energy. And thank goodness for that, as otherwise the recent observations of the professionals would be really hard to fathom. But the real beauty is that if, at some time in the future, they observe something else they can't explain, it will be apparent that its cause was created by the Big Bang too. Bloody brilliant! As theories go, they don't get much better than that!

You got my point exactly Nitpicker. Goodonumate!

by **Nitpicker** » Wed Oct 09, 2013 3:34 pm

The Big Bang presumably created matter, dark matter and dark energy. And thank goodness for that, as otherwise the recent observations of the professionals would be really hard to fathom. But the real beauty is that if, at some time in the future, they observe something else they can't explain, it will be apparent that its cause was created by the Big Bang too. Bloody brilliant! As theories go, they don't get much better than that!

On the other hand, the Cheerio-milk-vat model of the universe does not explain the angular momentum of galaxies, nor the acceleration of the expansion of the universe. Indeed, it cannot even explain the basic orbital motion of the Solar System. On the plus side, it might sustain one until lunch time.

Personally, as a rank amateur with a six-inch telescope, I have never made an observation which has required me to infer the existence of dark matter, nor dark energy. I am, however, quite comfortable with gravity, and I find it has a great many uses in my day to day life as well. Call me simple if you must.

by **BDanielMayfield** » Wed Oct 09, 2013 3:22 pm

Chris Peterson wrote:
BDanielMayfield wrote:But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.
However, dark energy is a fundamental part of the modern Big Bang theory, so in fact, the "Big Bang" does explain our observations.

Then the modern Big Bang theory, as modified to account for accelerating universal expansion, is a theory that explains observations, but it does so with another major, fundamental unknown plugged in, which is dark energy. As you stated yourself Chris, dark energy’s “actual nature remains very unclear.”

by **neufer** » Wed Oct 09, 2013 3:10 pm

Chris Peterson wrote:
BDanielMayfield wrote:
But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.
However, dark energy is a fundamental part of the modern Big Bang theory, so in fact, the "Big Bang" does explain our observations.

Einstein's full field equation:

is compatible with both the Big Bang
and the more recent discovery that the rate of universal expansion is now accelerating.

However, Einstein's full field equation was originally hypothesized on the assumption of a static universe with no Big Bang.

by **geckzilla** » Wed Oct 09, 2013 3:00 pm

BDanielMayfield wrote:What’s pulling the universe apart? This is one of the biggest mysteries of astrophysics.I could state what I strongly believe to be the cause, but doing so would be “frowned upon, in this establishment.”

You say it like it's a bad thing that we reject pseudoscience and fringe theories here. Chances are, if you have a strong emotional attachment to something, you aren't being objective, anyway.

by **Chris Peterson** » Wed Oct 09, 2013 2:41 pm

BDanielMayfield wrote:But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.

However, dark energy is a fundamental part of the modern Big Bang theory, so in fact, the "Big Bang" does explain our observations.

by **BDanielMayfield** » Wed Oct 09, 2013 2:33 pm

Nitpicker wrote:My simple understanding of the expanding universe is that the Big Bang caused it. Gravitational forces merely act on the resulting mass. The closer the masses are to each other, and the more massive they are, the greater the gravitational forces between them.

There are more complicated ways of thinking about it ... for example, I don't think I will ever understand neufer's Cheerio-milk-vat model.

But the Big Bang doesn’t explain the more recent discovery that the rate of universal expansion is now accelerating. If it was only gravity at work here then the universal expansion would be slowing down.

“Neufer’s Cheerio-milk-vat model” is an excellent analogy, as long as it isn’t taken too far. In the bowl or vat of milk surface tension causes the observed clumps of cheerios, but it also pulls the clumps against the sides of the bowl. In the cosmos gravity causes galaxies to cluster and merge, but the force causing the outward expansion to accelerate is unknown, and is called Dark Energy.

What’s pulling the universe apart? This is one of the biggest mysteries of astrophysics.I could state what I strongly believe to be the cause, but doing so would be “frowned upon, in this establishment.”

by **Nitpicker** » Wed Oct 09, 2013 2:12 pm

Neufer, sounds like your Cheerio-milk-vat model of the universe is closer to common-or-garden surface tension. And here I was thinking you'd developed a Grand Unified Theory.

by **Chris Peterson** » Wed Oct 09, 2013 2:02 pm

Nitpicker wrote:My simple understanding of the expanding universe is that the Big Bang caused it. Gravitational forces merely act on the resulting mass. The closer the masses are to each other, and the more massive they are, the greater the gravitational forces between them.

Correct. Two bodies acted only upon by their mutual gravitational attraction will separate forever, in the absence of any other forces, as long as their relative velocity is above their mutual escape velocities. The rate they separate will get ever slower, but separate they will.

That idea was behind one of the great cosmological questions that came out of the development of the Big Bang theories. Since no other force appeared to be involved besides gravity, would the Universe expand forever or would it eventually reverse and collapse again? The question depended upon determining if the bits were flying apart at greater than escape velocity.

Of course, recent observations have demonstrated that the rate of expansion of the Universe is actually increasing- something that can only be explained classically by a force opposite that of gravity. We call that force "dark energy", although its actual nature remains very unclear.

by **neufer** » Wed Oct 09, 2013 2:01 pm

wonderboy wrote:
this being the case, do they infact say "cheerio" as they move away from each other?

Is that what they say in Scotland

Because it all depends upon the Bond number... James Bond ().

http://en.wikipedia.org/wiki/Cheerios_effect wrote:
<<In fluid mechanics, the Cheerios effect is the tendency for small wettable floating objects to attract one another. An example of the Cheerios effect is the phenomenon whereby breakfast cereal tends to clump together or cling to the sides of a bowl of milk. This clumping behavior applies to any small macroscopic object that floats or clings to the surface of a liquid. This can include a multitude of things, including hair particles in shaving cream and fizzy beer bubbles. The effect is not noticeable for boats and other large floating objects because the force of surface tension is relatively small at that scale. (The Casimir effect, with a similar result, occurs at nanoscopic scale, and boats and other large objects floating in a choppy sea are subject to its classical equivalent; both are caused by waves, not surface tension.)

Writing in the American Journal of Physics, Dominic Vella and L. Mahadevan of Harvard University discuss the cheerios effect and suggest that it may be useful in the study of self assembly of small structures. They calculate the force between two spheres of density and radius R floating distance apart in liquid of density as

where is the surface tension, is a modified Bessel function of the first kind,

is the Bond number, and

is a nondimensional factor in terms of the contact angle .>>

by **Nitpicker** » Wed Oct 09, 2013 1:54 pm

Very impressive APOD by the way. Very sharp for a relatively narrow field of view observed through the atmosphere.

by **Nitpicker** » Wed Oct 09, 2013 1:38 pm

wonderboy wrote:this reminds me of a question i posed before! if gravity pulls things in, or towards then what is the equal and opposing force? it must be somthing that pushes things out, or away from it. the question i asked (i think) was: "is gravity responsible for expansion?" sounds silly but if Einsteins theory is correct then gravity must have an equal and opposing force.

Gravity was Newton. Relativity was Einstein.

In the case of two massive bodies, they each exert a gravitational pulling force on the other, that is equal in magnitude and opposite in direction, regardless of any difference in mass between the bodies. This is according to Newton's Third Law of Motion, which is applicable to any kind of force, not just gravitational.

My simple understanding of the expanding universe is that the Big Bang caused it. Gravitational forces merely act on the resulting mass. The closer the masses are to each other, and the more massive they are, the greater the gravitational forces between them.

There are more complicated ways of thinking about it ... for example, I don't think I will ever understand neufer's Cheerio-milk-vat model.

by **neufer** » Wed Oct 09, 2013 1:33 pm

wonderboy wrote:
this reminds me of a question i posed before! if gravity pulls things in, or towards then what is the equal and opposing force? somthing that pushes things out, or away from it.

If you are referencing Newton's third law:

3) For every action force there is an equal (in size) & opposite (in direction) reaction force.

The action force on NGC 3226 is an equal (in size) & opposite (in direction) to the reaction force on NGC 3227.

wonderboy wrote:
it must be the question i asked (i think) was: "is gravity responsible for expansion?" sounds silly but if Einsteins theory is correct then gravity must have an equal and opposing force.

If you are referencing Einstein's full field equation:

then the Einstein gravitational curvature tensor as defined by:

is curved one way by the matter/energy content of spacetime:

on the right side of the equation but
an opposite (though possibly unequal) way by the

cosmological constant on the left side of the equation.

by **wonderboy** » Wed Oct 09, 2013 1:16 pm

neufer wrote:
Markus Schwarz wrote:
Ron_2 wrote:
If galaxies are all being flung to the far-flung 'horizons' of the Universe, the outermost going fastest (?), how would they come to be on collision courses at all ?
The distance between galaxies is increasing, they are not "flung around". Though difficult to imagine this is something different than galaxies flying away from each other. The cosmic expansion makes itself felt on distances larger than about 400 Mpc, or about 1.3 Gly. The typical distance between galaxies is much smaller, say about 1 Mpc. Therefore, the interaction between galaxies is dominated by their mutual gravitational attraction.
I like to think about the universe as a large expanding (e.g., perhaps because the sides suddenly disappeared) vat of milk containing Cheerios.

Cheerios that are close enough together will condense into a permanent fixed sized clusters of Cheerios which move away from other permanent fixed sized clusters of Cheerios.

this being the case, do they infact say "cheerio" as they move away from each other?

Starship Asterisk*

APOD: Arp 94 (2013 Oct 09)

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