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Q: Accelrating Galaxies ( Answered 5 out of 5 stars,   1 Comment )
Subject: Accelrating Galaxies
Category: Science > Physics
Asked by: geoffreysteven-ga
List Price: $75.00
Posted: 11 Nov 2005 09:15 PST
Expires: 11 Dec 2005 09:15 PST
Question ID: 591921
It has been observed that galaxies and flying apart from each other at
an increasing rate. It seems that the more space between galaxies, the
faster they fly aprt. My question is this : Are they flying apart from
each other because the space itself is growing and increasing the
space between galaxies OR is it due to sub atomic particles popping
into and out of existance putting pressure on galaxies to force them
farther apart. OR is there another viable theory for the so called
"dark energy".
Subject: Re: Accelrating Galaxies
Answered By: guillermo-ga on 16 Nov 2005 00:46 PST
Rated:5 out of 5 stars
Hello Geoffreysteven-ga,

You know that in this field there are no certainties, but hypothesis
and theories that explain the observed phenomena as accurately as they
can, and that are more or less consistent with preexisting theoretical
background. Accordingly to the current explanations that the
scientific community is considering for further analysis and research,
"the space itself growing and increasing the space between galaxies"
(to use your own words as a point to start with) correctly expresses
the general idea of what seems to be happening in the universe. Also,
to be consistent with that assertion we shouldn't say that galaxies
"fly apart from each other", but that the *space* in between is

Everything begins with Einstein's Theory of General Relativity:

"General relativity (GR) is the geometrical theory of gravitation
published by Albert Einstein in 1915. It unifies special relativity
and Isaac Newton's law of universal gravitation with the insight that
gravitation is not viewed as being due to a force (in the traditional
sense) but rather a manifestation of curved space and time, this
curvature being produced by the mass-energy content of the spacetime."
(General relativity: )

So far, what we basically need to highlight for our purpose is that
there is a relation between the shape of spacetime and gravitational

Further development of this theory done by Belgian astronomer (by the
way, Catholic priest) Georges Lemaître, led to his "hypothesis of the
primeval atom" first published in 1927 that, briefly, explained that
all the matter, energy, space and time in the universe originated from
a *singularity*, an infinitely hot and small concentration that
exploded in all directions forming - in time - our current,
necessarily expanding, universe.

Despite the consistence of Lemaître's work with the theory of general
relativity, Einstein - who envisioned an immutable and non-expanding
universe - did not accept that proposal immediately. Persuaded as he
was of a stationary universe Einstein had conceived a "cosmological
constant" ( ) that
would correct his own theory's equations that proved inconsistent with
a stable universe. Confronted to the fact that the cosmological
constant would not - ultimately - help the theory of general
relativity fit with a stable universe, and that Edwin Hubble's
observations confirmed that the universe was expanding, Einstein
enthusiastically embraced Lemaître's theory, and rejected his own
cosmological constant as his "biggest blunder".

However - like we'll see later - that "blunder" is in present days of
most interest, and directly related to this question, as it is being
considered among the attempts to *explain the recently discovered
acceleration of the universe expansion*.
( )

In 1948 Fred Hoyle proposed a model known as "steady-state universe",
which attempted to be consistent with the Hubble law of an expanding
universe while postulating an ever existing universe (as opposed to a
"primeval atom") by supposing the constant generation of new matter
( ). Searching to
disqualify Lemaître's model, Hoyle nicknamed it "Big Bang", by which
ironically Lemaître's theory was since then known.

Later, the observation of Cosmic Microwave Background - a radiation
dating from the origin of the universe consistent with the Big Bang
theory, led the scientists to dismiss the steady-state universe theory
- at least for now (we never know in physics...)

So, summarizing so far:

- General relativity relates spacetime shape with mass-energy gravitation;
- Einstein tries to make the idea of a stable universe fit his theory
with a cosmological constant that later rejects - but which is now
vindicated in an attempt to explain the acceleration of the
intergalactic space stretching;
- The Big Bang theory postulates a primeval atom (consistent with
general relativity's singularity) and an expanding universe;
- Hubble observes expansion and calculates that the speed of the
receding galaxies is proportional to their distance, in other words,
the farther, the faster, consistently with an homogeneous universe.
Please notice that this increase of speed as galaxies are farther -
enunciated by Hubble, and expressed in the Hubble's constant - *is
not* the acceleration that in recent years was observed and urged for
new hypothesis, which is besides Hubble's constant, and actually
challenges it. ( )

Now, questions emerged such as: Will that expansion last forever? Will
it remain constant? Will it eventually recede? Will it increase?

Regardless how counterintuitive that may seem, that is a matter of
geometry, and that is because of the general relativity. It postulates
that "gravitation is (...) a manifestation of curved space and time,
this curvature being produced by the mass-energy content of the
spacetime" ( ). As an
example of this, massive bodies such as stars or planets curves the
spacetime around them, causing other bodies, particles, even light to
approach them.

Consequently, the total amount of mass-energy in the universe
determines the curvature of space for the whole universe.

Depending on the total amount of matter and energy in the universe,
its geometry can be flat, spherical or hyperbolic. This doesn't mean
that our universe is either a plane, a sphere or a saddle, as the
typical images used to outline these concepts may mislead to believe
if not sufficiently explained (the shape of the universe itself is
much more complex, and is a *topological* problem). What the geometry
of the universe means is that the space can be:
- flat: then its geometry will behave like on a flat surface - two
parallels will remain parallels infinitely, and the sum of the inner
angles of a triangle will equal 180º;
- spherical: then its geometry will behave like on a spherical surface
- two parallels will eventually converge and the inner angles o a
triangle will sum more than 180º;
- hyperbolic: then its geometry will behave like on a saddle-shaped
surface - the distance between two parallels will eventually increase,
and the sum of the inner angles of a triangle will be less than 180º.

Alan Guth's inflationary theory of the universe, meant to address some
flaws of the big-bang theory, also explains the geometry of the
universe being extraordinarily flat. The density of matter-energy
needed to keep the universe flat is known as *critical density*. For
lower density values, the universe should have a hyperbolic space, and
for values higher than the critical, the space geometry would become

But when a census of the total matter-energy density of the universe
proved it to be much less than expected for a flat universe,
scientists resorted to an invisible or "dark" matter that since the
1930s had been proposed to explain, for example, the orbiting of stars
around galaxies and the latter around galaxy clusters. This *dark
matter*, supposedly consisting of particles yet to be unveiled, was
expected to account for the missing 95% needed to reach the critical
density. In spite of this some measurements, while confirming dark
matter's existence, also showed that in much less quantity than
expected. Thus, the total amount of matter of all kinds in the
universe is less than half the critical density.

This could have challenged the acceptance of the flat universe
predicted by the inflation theory, if new observation hadn't brought
up more evidence in favor of a flat universe. The *cosmological
microwave background* is a radiation left over from the big bang,
which precise measurements in the early 1990s strongly supported the
inflationary predictions.

(Later, in 1999, the ground-based MAT/TOCO experiment, and in 2000 the
balloon-based BOOMERANG and MAXIMA experiments with better precision,
again proved consistent with a flat geometry.)

Thus, the problem of the shortage of matter-energy to reach the
critical density needed to keep a flat geometry still remained. In the
mid 1990s, several groups, including Jeremiah Ostriker and Paul J.
Steinhardt from Princeton University, proposed to attribute the
missing two-thirds of the critical density to an exotic "dark energy",
different from the dark matter in its properties.

However, while this concept explained very satisfactory this and other
problems, it also brought out that to completely fit the equations, it
had to be *gravitationally self-repulsive*. Ultimately, this meant
that if the proposal was correct, *the dark energy would cause the
expansion of the universe to accelerate*, i.e. contrary to the
accepted knowledge at that time, the acceleration had to be occurring
if the dark energy concept was correct.

"Then, in 1998, two independent groups - the Supernovae Cosmology
Project and the High-Z Supernova Search - announced a spectacular
result based on significantly more precise measurements of cosmic
expansion. Their observations of the brightening and dimming of
distant type 1a supernovae revealed that the expansion of the universe
is in fact accelerating."

This strongly supportive observation (one of the major discoveries in
astrophysics of our time, by the way), led to the consequent
challenge: what is this *dark energy* that constitutes the two-thirds
of the energy in the universe. It has negative pressure - what causes
the acceleration of the expansion - resists the gravitational pull of
galaxies. To the question about what particles is it made of,
speculations arise on it having no particles at all.

Some scientists have resurrected the *cosmological constant*, the
"Einstein's blunder" we saw in the beginning, which was a form of
energy with constant negative pressure, based on vacuum energy.

Another, brand new concept - that holds the ancient name of
*quintessence* - was proposed by Rahul Dave in 1998, then at the
University of Pennsylvania. "Quintessence is a dynamic, time-evolving
and spatially dependent form of energy with negative pressure
sufficient to drive the accelerating expansion. Whereas the
cosmological constant is a very specific form of energy - vacuum
energy - quintessence encompasses a wide class of possibilities.

"The simplest model proposes that the quintessence is a quantum field
with a very long wavelength, approximately the size of the observable
universe. Some examples had been explored almost a decade earlier by
Bharat Ratra and James Peebles at Princeton University, and by Chris
Wetterich at the University of Heidelberg in Germany. A particle is
usually thought of as a bundle of oscillations in a quantum field, but
since this bundle is much larger than any conventional length scale,
the particle description is impractical."

(, Article: "Quintessence", by Robert R Caldwell & Paul
J Steinhardt, November 2000 )

Summarizing in accordance to your question:

"It has been observed that galaxies are flying apart from each other
at an increasing rate. It seems that the more space between galaxies,
the faster they fly apart. My question is this : Are they flying apart
from each other because the space itself is growing and increasing the
space between galaxies OR is it due to sub atomic particles popping
into and out of existence putting pressure on galaxies to force them
farther apart. OR is there another viable theory for the so called
'dark energy'."

Your question refers to two different phenomena - both depicted by
science - which we'd better discern:

- The galaxies separating from each other at an increasing speed as
the go farther is a long time accepted phenomenon observed and
explained by Edwin Hubble in 1929, and expressed by Hubble constant;

- The recently discovered acceleration of that expansion - first,
predicted by the theory of dark energy, and later confirmed by
observation - is a totally different phenomenon, and I guess is the
one you were more interested about;

- The expansion depicted by Hubble occurs because "the space itself is
growing and increasing the space between galaxies" as you said;

- The acceleration recently identified is attributed to the *dark energy*;

- The two current proposed explanations for dark energy are the
resurrected Einstein's *cosmological constant* and the concept of

- There seems to be no subatomic particles involved in these phenomena.

I hope this answer satisfies your expectations. Please don't hesitate
to ask for clarification if you find it necessary. Thank you for such
an interesting question.

Best regards,

geoffreysteven-ga rated this answer:5 out of 5 stars

Subject: Re: Accelrating Galaxies
From: guillermo-ga on 16 Nov 2005 10:10 PST
Hello Geoffreysteven-ga,

Thank you very much for the 5 stars! :)



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