Expert: Tom Whiting - 7/23/2008

Question
The General theory of relativity was written based on a static universe without taking the big bang into account. So the heavy masses attract the lighter object and keep them in orbit around them with the help of their gravitational pull. If there were more objects, than due to their collective gravitational attraction the masses should attract each other and collide unlike objects put in the orbit. Einstein introduced that there should be an equal and opposite force acting on these objects to compensate gravity which keeps the masses in orbit.
He called it as Cosmological constant.
Now the according to bigbang the universe is expanding.
My question is, could this cosmological constant be the effect due to expansion of the universe. Like a police chasing a thief, the expansion might keep the masses falling together due to the gravitational attraction. I mean there is a balance between these two, which Einstein called it as cosmological constant.
And if the big bang expansion stop what will happen to these masses, will the planets be in orbit around the Sun or attracted towards the Sun
Thanks for your answers in advance !

/Mohan

Answer
Hi Mohan,
Realize right off the top that just because Albert Einstein believed
in a static Universe, that does not make his general relativity
principles wrong. He just mistakenly set the cosmological constant
at zero. (He later claimed that it was the biggest mistake he ever
made.) We now know that it is not zero any more, so his idea
of gravity being a curvature of the space-time continuum still holds.

In the early Universe, gravity was slowing (decelerating) the expansion (well after the inflationary phase), but then about 6 or 7 billion years ago, the expansion began to accelerate, today we think due to the presence of dark energy (72%), and it's expansion is accelerating more and more as time passes.
So your question as to "if the expansion stops" is irrevelant now, because.... it won't ever stop expanding; IN FACT, the expansion is accelerating with time.
(See recent Type Ia supernovae results).

So the masses won't ever "fall together". And there will be no "Big Crunch". The Universe will end hundreds of billions of years from now in either a 'heat death' or Big Rip...it's either one or the other; a third result, (if) protons are unstable with a half-life of some 10 to the 32nd power years as has been suggested, is just too far....down the road.  Other 'endings' occur way before that event.

So the cosmological number is now due to the presence of somethings
we call dark energy (72%) and 23% dark matter...we don't know what  either one are, at present. But we do know that both must exist because of the recent COBE and WMAP results. See
http://map.gsfc.nasa.gov/news/
This is probably not the final word, as we learn more and more.
But the future accelerating Universal expansion (of the space-time
continuum) is pretty much carved in granite now.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA

PS...you used the terms 'gravitational pull' and gravitational force,
in your question. Neither define gravity anymore (Those were Sir
Isaac Newton's incorrect concepts of gravity); Einstein covered
all that with his General Relativity, circa 1917...gravity is neither a pull nor a force, but simply a curvature of the space-time continuum due to the presence of a mass. Just thought we'd better
clear that up now.

ADDITIONAL FOLLOW UP:
You mentioned spacial expansion and planetary orbits in the same
question. The Universal expansion is only 71 Km/second per Megaparsec, (Hubble's constant) or 42 miles per second, PER 3.2 million light years!
This is such a small value, unless you're dealing with multi-million
light year distances, it's not even measureable on the scale
of the Solar system, or any planetary scale, or even at our entire
Galaxy scale of only 100,000 light years. And it's just the space
expanding at that slow rate, not the matter itself.

FOLLOW UP:
Sorry for the delay....I didn't see that you had an additional
question formulated in your rating answer....next time it's just
best to do a follow up question as opposed to including it in the
rating answer. (Otherwise, it's easy for us to miss it).

Well, complete understanding of string theory could go a long way
in comprehending a lot of stuff. But I think we are capable of
other methods in the very near future of explaining dark matter
and dark energy. We may not be that dependent on string theory
for that one. We just don't know as yet.

Yes, I also envison the initial big bang as basically a collision
or joining of two very massive particles too. But with respect to
multi-dimensional (or more properly, multi-existing) Universes as a cause, that's not science as there is no way to test, gather data, observe, or repeatability (which is science) of "other Universes", so that may always be in the realm of philosophy. We in science just cannot know. (Even science cannot know the unknown, as Mr. Spock used to say on Startrek).
As far as 11 dimensions in our Universe, that's only at the sub-atomic quantum level, not our usual macro level. We only have 3 spacial dimensions and time at the macro level, and never the two shall meet (in my opinion). In other words, wormholes and time
holes and stuff like that are only a quantum property which don't
occur on the macroscale. We have to keep science fiction separate
from science fact.
Hope this helps,
Clear skies,
Tom Whiting
Erie, PA