Cosmic entropy could be 100 times greater than previously thought

October 6, 2009 by Lin Edwards, weblog

( -- A new analysis of supermassive black holes has discovered the entropy of the universe is much greater than previously thought, which means it may also be very slightly closer to ultimate heat death.

The study, analyzing measurements of the mass function, was carried out by two Australian cosmologists: Chas A. Egan from the Australian National University in Canberra, and Charles H. Lineweaver from the University of NSW in Sydney. Their results indicated the entropy at the current cosmic event horizon is much greater than the entropy of the interior of the , and the total cosmic entropy is about 100 times greater than previous calculations.

Entropy increases as the number of ways the system can be arranged microscopically without changing the external appearance increases. Egan used the example of hot water being poured into a cold bath. Before the hot and cold water meet they are separate and orderly and the system has low entropy. When the hot and cold water are well mixed, the entropy is high and no between the two is possible.

Egan and Lineweaver found the collective entropy of the supermassive black holes was around 100 times higher than expected. Since these black holes contribute more to the entropy in the universe than anything else, the results imply that the entire cosmic entropy is also a 100 times greater than earlier estimates.

Previous calculations of the cosmic entropy assumed the presence of a 10 million solar-mass black hole at the center of each galaxy, and the entropy was calculated using an estimated average mass. Egan and Lineweaver had access to more recent data that gave them the range of supermassive black hole masses rather than an average. Egan said the study revealed a smaller number of larger supermassive black holes contribute much more to the entropy than previously believed.

The universe has much lower entropy than is theoretically possible, and this is still true, even with the new calculations. This is just as well because the entropy of the universe must be below the maximum theoretical value or life and other complex phenomena will cease to exist. As the entropy gradually increases it will eventually approach the theoretical maximum, a state many physicists have called the heat death of the universe. The new calculation takes the universe a little closer, but it is still only a billionth of a billionth of the maximum.

Not every scientist agrees that the higher entropy takes the universe closer to heat death. Ned Wright of the University of California in Los Angeles, for example, suggests the entropy is locked inside the supermassive , and so the rest of the universe has lower and is therefore further away from heat death.

More information: The paper was published online at


© 2009

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Oct 06, 2009
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1 / 5 (1) Oct 06, 2009
When massive body evaporate into radiation, entropy increases, when they condense by gravity, entropy decreases. Does total entropy of Universe increase or decrease, after then? I don't think, entropy stuff is relevant for description of Universe evolution at all.
Oct 06, 2009
This comment has been removed by a moderator.
not rated yet Oct 06, 2009
Entropy increases as the number of ways the system can be arranged microscopically without changing the external appearance increases.

So what's the external appearance of the universe? For thermodynamics to be applied to a system, doesn't it need to be limited? What are the limits of the universe, then?
5 / 5 (3) Oct 06, 2009
When massive body evaporate into radiation, entropy increases, when they condense by gravity, entropy decreases.
Entropy increases when they condense as well.
Does total entropy of Universe increase or decrease, after then?
The universe is a closed system, it has a finite density; therefore, it has a finite entropy density. This entropy density tends to increase with time.
Additionally, what is being discussed in the article is the entropy within the cosmic horizon, i.e. the entropy within the volume of space from which light has been able to reach us.
1 / 5 (1) Oct 07, 2009
..what's the external appearance of the universe..
Has such question meaning, if we can never observe Universe from outside (being insintric part of it)? The occasional answers can never be tested.
not rated yet Oct 07, 2009
i.e. the entropy within the volume of space from which light has been able to reach us.

So that would be the limit of the system, as we treat it. But what about the light that couldn't, or hasn't reached us?

I know it's not practical to ask such questions, but the thing is, cosmology isn't really much of a practical science. I think there are too many assumptions and approximations based on practicality and technological limitations.

Since there are currently few, if any, applications for this field, the focus should be on being as unequivocal as possible.
1 / 5 (1) Oct 07, 2009
It is an untested hypothesis that black holes are supermassive, and therefore, coming to a conclusion about entropy using this hypothesis is also an assumption.
not rated yet Oct 11, 2009
Are they saying that supermassive black holes are evaporating 100 times faster than previously thought?

How can you possibly "measure" that?

Or are they saying that the black holes are larger than expected?

How can a black hole have entropy unless something is falling into it, or evaporating out of it? Isn't a black hole literally frozen in time? This article is very confusing.
5 / 5 (1) Oct 11, 2009
i.e. the entropy within the volume of space from which light has been able to reach us

This area is a great deal smaller than the "known" size of the universe depending on the complete interpretation of that phrase.

The expanding universe has much of what we have observed (i.e. distant galaxies) being now forever outside the volume of space where any light from those sources produced today could ever reach us.

Let us not forget that current measurements have the universe expanding at many times the speed of light.

So the entropy measured would be the either the entropy in our current region which includes heat from the distant past or it is the entropy from the measured "known" universe which is younger the further away the light was when it started.

It is just that are we calculating the entropy as it is now everywhere or as we measure it from sources that may have originated at or near the birth of the universe, and would therefore have barely started towards entropy.
5 / 5 (1) Oct 12, 2009
The entropy of a black hole is proportional to the surface area of its event horizon. Entropy is, by definition, k*ln(O) where k is Boltzmann's constant, and O is the total number of microstates in which the object can exist. Thus, the entropy of a black hole can be calculated by determining the number of (unique) possible ways one can produce the observed properties of said black hole. The entropy of the universe has been estimated using the known entropies associated with various (astronomical) objects and the expected number within the visible universe. The maximum possible entropy of the visible universe is obtained by assuming the visible universe would have the highest entropy density possible, that of a black hole with a radius equal to the distance to the edge of the visible universe (I think). This article reports on the determination of a better value for the expected number of black holes in the visible universe as a function of mass (at least for supermassive BH's).

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