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Astronomy/Accelerating Universe



Please forgive me, I barely passed high school although I'm a professional pilot. 61 y.o.

My question:

Can the accelerating universe be explained by the fact that it must be becoming less dense from that expansion?
If time changes near mass and the universe is becoming less dense, maybe it just looks like it's accelerating because time is changing and it's not acceleration after all.

Not sure how to word that but I bet you know what I'm trying to say.


ANSWER: Hello, and thanks for your question.

Actually, to the extent we trust our measurements (including for the density of the cosmos, red shifts etc.) and we do, the acceleration is real and is tied to what we call dark energy.We start with the basic relation for the parameter OMEGA - the ratio of the measured density (rho) to the critical density:

Omega = rho / rho_c

(where the denominator refers to the critical density of the cosmos.) Thus if:

rho (greater than) rho_c

(c = critical)

then the cosmic density is able to reverse the expansion (e.g. decelerate it) and conceivably usher in a new cycle. (New Big bang etc.) The observations that help determine how large rho is, come mainly from observing galaxy clusters in different directions in space and obtaining a density estimate from them.

Current data, e.g. from Boomerang and other satellite detectors shows that  OMEGA~ 0.3 or that:

rho  = 0.3 (rho_c),   i.e. barely three tenths of the critical value to decelerate

Thus, since  rho <  rho_c  there is no danger of the cosmos decelerating.

This is based on multiple source measurements.((See, e.g. 'Supernovae, Dark Energy and the Accelerating Universe', by Saul Perlmutter, in Physics Today, April, 2003, p. 53.)

For a fairly readable account, please get hold of Lawrence M. Krauss' book, 'A Universe From Nothing', and comsult Chapter 5, 'The Runaway Universe'.

He shows very convincingly the acceleration is quite real and not from any illusion or mere appearance.

---------- FOLLOW-UP ----------

QUESTION: Ya know, I actually understand that. Not so much the formulas. Thank you. It makes me wonder, is dark matter condensed dark energy? A different form of the same thing?

Thanks again.


No, dark energy is totally novel. Thus, dark energy is a separate concept from dark matter. The former is based entirely on what we call an equation of state, written

w = (p / rho) = -1

Without going into a lot more math, this is consistent with Einstein's general theory of relativity - which one could say approaches the status of a 'basic law of physics'. In this case, a *negative pressure* (if you solve for p you will see what I mean)  meshes with general relativity's allowance for a repulsive gravity since any negative pressure has associated with it gravity that repels rather than attracts.

Some might argue that such cosmic repulsion shows a "new law" of physics, but it's merely extending the existing concept of gravitation to show it has a repulsive as well as attractive aspect, and has always been consistent with Einstein's general theory of relativity.

Dark energy then emerges as a source of repulsive gravity and accounts for the cosmic acceleration.

Again, I believe that Chapter 5 in Krauss' book can be of immense help here.


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Philip Stahl


I have more than forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics pertaining to sunspot morphology and flare geo-effectiveness.


Astronomy: Worked at university observatory in college, doing astrographic measurements. Developed first ever astronomy curriculum for secondary schools in Caribbean. Gave workshops in astrophysics and astronomical measurements at Harry Bayley Observatory, Barbados. M.Phil. degree in Physics/Solar Physics and more than twenty years as researcher with discovery of SID flares. Developed of first ever consistent magnetic arcade model for solar flares incorporating energy dissipation and accumulation. Develop first ever loop solar flare model using double layers and incorporating cavity resonators.

American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union.

Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'. 'Astronomy and Astrophysics: Notes, Problems and Solutions'.

B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research (1980), Barbados Astronomical Society Award for Service as Journal Editor (1977-91)

Past/Present Clients
Caribbean Examinations Council, Barbados Astronomical Society, Trinidad & Tobago Astronomical Society.

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