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Mark Perakh's Web Site

Cooling of the universe

Pseudo-thermodynamics of Hugh Ross revisited

By Mark Perakh

Posted May 13, 2004

On April 26, 2004 I received an email message which is reproduced here in its entirety:

Dr. Perakh,
I recently picked up a copy of your Unintelligent Design and wanted to alert you to some errors.  As a brief introduction, I am a graduate student in Astronomy and have followed the intelligent design movement for several years. I find debunking such creationist claims an entertaining mental exercise and overwhelmingly approve of the purpose of your book.
On pages 173-178, you discuss Hugh Ross's treatment of thermodynamics in a cosmological setting. You correctly point out that Ross's treatment is not overwhelming rigorous--however, his claim that the universe cools because it is expanding is correct.
Ross makes the (regrettably) common slip of referring to an adiabatic expansion when he really means a reversible adiabatic expansion. Because it is assumed that the universe remained in thermal equilibrium throughout much of it's early history, the expansion does indeed cause it to cool.
A couple statements of yours that really need to be changed: on p. 177: "According to the second law, the entropy of the universe increases along with the latter's expansion." This is a misstatement. The second law says entropy increase OR stays constant. The entropy of the early universe, (and I think the latter universe) is dominated by the background radiation which is expanding isentropically.  When measured on cosmological scales, entropy is constant.
On p. 178: "The temperature drop occurs because the initial thermal energy of the universe gradually converts...into the kinetic energy of the motion of those clumps of emerging matter moving away from the initial seed of the universe." Again, the cooling is indeed caused by the expansion. The end of this statement hints at some muddled thinking about how the expansion proceeds--matter is not flying apart from some initial center in big bang cosmology. I have a hard time picturing how thermal energy could be converted into the proper motion of galaxies--it makes a lot more sense for that motion to come from gravitational collapses and cosmological torques.
I believe Ross has very selectively gone through the scientific literature to pick out bits that support his theology, but his science is usually correct.
By saying his statements are "preposterous" and that he doesn't understand "elementary concepts" when his explanation is much closer to correct than yours greatly undermines your credibility.
I would recommend looking into a cosmology text for how the thermodynamics are treated. Peacock's Cosmological Physics is popular, I tend to fall back on Matt Roos' Introduction to Cosmology.

Peter Yoachim

While I disagree with Yoachim's remarks addressing my critique of Hugh Ross, (and I will show here, why), I appreciate his comments.

The purpose of writing up this brief article is not so much to respond to Yoachim as to clarify for a wider audience some points raised in his message. If Yoachim seems confused by some points in my book, so may others be, and hence some crude errors in the very popular publications of Ross apparently require a little more detailed explanation. (Besides my book referred to by Yoachim, a similar critique of Ross's misinterpretation of thermodynamics also is found in my paper titled "Intelligent Design For Dummies," which was printed in The Skeptic magazine (Australia), vol. 23, No 4, pp 22-27, as well as on this site at www.talkreason.org/articles/ross.cfm; this article therefore should serve as a clarifying addition to all three listed sources).

Since, however, this discussion was set into motion by Yoachim's letter, I'll briefly address his critique per se.

First, note the ellipsis in his quotation from my book, when he writes "universe gradually converts... into the kinetic energy of the motion of those clumps of emerging matter moving away from the initial seed of the universe." Using ellipsis is often a device aimed at changing the contents of a quotation to fit the agenda of the critic. It also is referred to as quoting out of context (although, since Yoachim's was a personal message to me, he obviously did not intend to distort my position).  

The full text of the pertinent paragraph in my book makes it clear that I suggested a hypothetical explanation of the universe's cooling – by stating that it occurred because the thermal energy of the universe's "seed," starting at Planck's time, converted into non-thermal forms of energy. As candidates for those non-thermal forms of energy I suggested several possible alternatives – the rest energy of the emerging masses, the kinetic energy of the moving masses, etc. I did not suggest any specific mechanism of the conversion of thermal energy into non-thermal forms, but based my explanation only on the law of energy conservation.

Unless we choose to discard the law of energy conservation (and I am not categorically rejecting the possibility of such an approach, especially if the discussion relates to cosmology and general relativity) we have to assume that thermal energy of the emerging universe converted into non-thermal forms thus resulting in the universe's cooling. This is a very general statement not precluding references to any specific phenomena which could be instrumental in the universe's cooling.

Yoachim writes that he has a hard time imagining the conversion of thermal energy into kinetic energy of masses moving away from each other. This is the argument from ignorance which has no sufficient cognitive weight. We may not know the precise mechanism of the conversion in question, but if we choose to adhere to the law of energy conservation, the mere fact of such conversion is hardly deniable.

Now turn to Yoachim's defense of Ross, who, in Yoachim's view, is correct when stating that universe cooled because of expansion. Let us recall exactly how Ross renders his explanation.

On page 135 of his book Creation and Time Ross writes:

"As the universe expands from the creation event, it cools, like any other system obeying the laws of thermodynamics. When the heat energy of a system fills a greater volume, there is less energy per unit volume to go around." 

As was mentioned in my book, that statement is preposterous.

Note that Ross, in this quotation, refers to thermodynamics rather than to cosmology.  Ross's statement does not mention such points as the red shift of photons, cosmic scale factor, or any other points that one may see discussed in the literature on cosmology. His argument is fully within the framework of classical thermodynamics.  And as such, it is egregiously wrong.

Indeed, "heat energy" -- the term used by Ross which evidently was meant to denote "thermal energy"  -- is not a substance that can be diluted in a certain volume so that "the greater the volume the less energy density." Such a view could have sounded plausible until the end of the 18th century, i.e. until Benjamin Thompson (Count Rumford) understood that "heat" is a form of motion. In the 19th century, physics clarified that thermal energy (this concept makes sense only for macroscopic systems) is in fact the kinetic energy of the particles of which the body consists. Cooling of the universe since the big bang has nothing to do with the non-existing effect imagined by Ross – the decrease of energy density when the same amount of "heat energy" spreads over an increasing volume.

Since my task was to show the errors of Ross's discourse rather than explain why the universe cooled, I could stop discussing this point right here without going into the explanations of the universe's behavior. Since, though, Yoachim introduced this question in his letter, I'll briefly relate to it here in a little more detail than I did in my book, my paper in the Skeptic, and in the post on this site.

First, though, I'll discuss Yoachim's remark about entropy.

While in my book I wrote that the entropy of the universe increases along with its expansion, Yoachim thinks that entropy remains constant (the 2nd law allows for the entropy in a closed system either to increase or to remain constant).

Let us turn again to classical thermodynamics (because Yoachim cites here the 2nd law of thermodynamics rather than some cosmological argument). Entropy, according to the 2nd law, can indeed remain constant but only either in reversible adiabatic processes or in reversible cycles. Classical thermodynamics asserts, though, that all real processes in macroscopic systems are irreversible. Classical thermodynamics further asserts that in irreversible cycles the net entropy always increases rather than remaining constant. Therefore, whatever happens in the universe, its net entropy constantly increases (remember that this argument is made without entering cosmology). This led Boltzmann to his pessimistic prediction of the imminent thermal death of the universe. Boltzmann did not know about universe's expansion, which requires a reconsideration of his prediction.

Boltzmann was very instrumental in understanding that entropy is in fact a monotonic function of the number of accessible states. As the universe expands, more accessible states emerge. According to the modern view, at Planck's time the universe's entropy had the maximum possible value for the situation existing at that moment -- the embryonic universe was completely disordered. As the universe expanded, two tendencies came into play: gravity worked towards creating islands of order thus causing local decreases of entropy, while expansion generated new accessible states thus enabling a continuing increase of the net entropy of the universe above the level it had at Planck's time.  

In regard to the universe's expansion, is it legitimate to say that expansion caused cooling? It depends on the framework of our discussion. Remember that Ross stays within the framework of thermodynamics -- so let us stay within thermodynamics as well when discussing Ross's statements. Let us recall that there are many known processes where a thermodynamic system expands but temperature does not drop.

Consider conventional systems such as discussed in classical thermodynamics, say, a cylinder with a piston, filled with a gas.

The concept of a system in thermodynamics incorporates as ineliminable parts concepts of a boundary and of the surrounding. The boundary separates the system from the surrounding. The boundary may be adiabatic -- that is impervious for a heat transfer to or from the surrounding. It can be diathermal, which is opposite of adiabatic -- it allows a free influx to or outflow from the system, of heat. It can be anything in-between.

Assume the gas in the cylinder expands. Does it mean the gas is necessarily cooling down in the process of expansion (as Ross's assertion implies)? Not at all. The temperature of gas in the course of expansion may increase, remain constant, or decrease. It depends on what happens at the system's boundaries. If there is heat influx into the system through its boundaries (in this example through the cylinder's walls) the gas's temperature may go up despite its expansion. The expanding gas performs work against an external force -- in this case the force is the weight of a load placed on the piston – at the expense of the gas's internal energy. If the amount of work exceeds the heat influx to the gas from the surrounding, gas's temperature will decrease. This decrease is, though, caused not by expansion as such but by the negative balance of energies: if no work is done, temperature will not go down.

Indeed, imagine a box divided by a partition into two compartments. Fill one compartment with gas at temperature T and pressure P. Keep the other compartment vacuumed (P=0). Rapidly withdraw the partition. Gas will expand into the empty compartment and this will be a very fast process. It results in the system's entropy increasing. However, gas's temperature remains the same -- expansion into vacuum is an isothermal process.

This completely negates Ross's ridiculous assertion that "when the heat energy of a system fills a greater volume, there is less energy per unit volume to go around."  (BTW, because of the very high speed of the expansion into vacuum, this process also is adiabatic -- which negates another incorrect statement by Ross, as discussed in my book).

In the example with a piston in a cylinder, gas would cool down only if it performed work against external forces -- for example against the weight of a load placed on the piston -- and if the expenditure of the gas's internal energy on doing that work is not compensated for by the heat influx from the surrounding. In the case of expansion into vacuum there are no external forces -- the empty compartment does not exert any resistance to the expanding gas, so there is no decrease of the gas's internal energy of which temperature T is a measure -- and T does not decrease.

Since expansion itself is not the cause of cooling -- there are many processes where a system expands but no cooling occurs -- Ross's assertion, which states that according to the laws of thermodynamics expansion causes cooling, is utterly misleading.

Go back to the universe. Extending classical thermodynamics to the universe requires caution. The concept of a thermodynamic system makes no sense without the concepts of a boundary and of the surrounding. When we talk about the universe these concepts lose their standard interpretation as used in classical thermodynamics. There supposed to be "nothing" beyond the universe -- no "surrounding" and therefore no boundaries in a thermodynamic sense. That is why it is often claimed that the universe is a perfect closed system -- it does not exchange either matter or energy with a surrounding because there is no surrounding. The universe expands not into vacuum, as gas into an empty compartment, but into nowhere.

If we don't want to easily discard the law of energy conservation -- which would be a rather non-parsimonious approach -- we have to attribute the universe's cooling to the conversion of thermal energy into non-thermal forms of energy. As to specific forms of non-thermal energies which are beneficiaries of the conversion, their relative contributions to cooling, and specific mechanisms of conversion -- here is a space for various hypotheses and models.

In cosmology the cooling of the universe is often attributed to the cosmological red shift of photons. (There seem to be various cases of red shift. One is the red shift of light coming from remote galaxies which is usually attributed to Doppler's effect and obeys Hubble's law. Another is the gravitational red shift caused by the space-time's curvature which is due to the presence of large masses. One more is the cosmological red shift caused by the expansion of the space-time per se. General relativity, however, reveals that all three cases, if explained by the properties of the metric tensor, differ only in interpretations based on various approximations and can all in fact be attributed to space-time's expansion.)

 As an illustration, this cooling effect can be juxtaposed with what is called Wien's displacement law. It relates to the spectra of black body radiation. The dependence is as follows:

T= K/λmax

where T is temperature and λmax is the wavelength corresponding to the maximum of the spectrum (i.e. the most common wavelength in the radiation).

K=2.898x10-3 Kelvin-meter is Wien's constant.

In the course of the universe's expansion, the wavelength of radiation increases together with the expansion of space-time (that is the red shift mentioned above). This, according to Wien's law, means a temperature drop. It is in this sense that cosmologists say that expansion causes cooling.

However, Ross's discourse has nothing to do with the red shift mentioned. Ross attributes the universe's cooling to its expansion, in purely thermodynamic terms (suggesting a ridiculously wrong mechanism). Since expansion of thermodynamic systems, in general, is not necessarily accompanied by cooling, assertions regarding cooling of the universe because of its expansion, in the form Ross makes them, are egregiously wrong.

From another standpoint, pointing to the red shift and the concomitant spectrum's shift toward lower temperature does not contradict the more general statement which refers to the law of energy conservation and the transformation of thermal energy into non-thermal forms. Therefore, since Ross appealed only to thermodynamics but not to cosmology, and his thermodynamic explanation (of energy diluting in increasing volumes) is absurd, my critique of Ross (which was entirely about his use of thermodynamics) is valid.  

Perhaps we might note that the law of energy conservation is not clearly understood within the framework of the general theory of relativity and cosmology. While cosmologists usually have no problem with applying laws of physics, as they are known under the earthly conditions, locally, they face uncertainty when these laws are applied to the universe as a whole (which is connected to the curvature of space-time).  The law of conservation of energy is no exception. However, this peculiarity of the cosmological interpretation of the energy conservation law is not negating the explanation of the cooling effect which attributes it to the conversion of thermal energy into non-thermal form. Indeed, this explanation does not require extending the conservation law to the universe as a whole. Acts of conversion occur locally and that is where the law of conservation of energy works in cosmology as it does anywhere else. Also, the uncertainty related to the extension of the law of energy conservation to the universe as a whole does not mean that the law in question is invalid for the universe as a whole – it only means that there is a possibility that the law in question may need modification if applied to the universe as a whole. Except for some special discussions limited to the narrow points of cosmology, the law of conservation of energy is invariably construed as one of the most fundamental postulates of physics (although the concept of energy itself is one of the least understood concepts in physics).

My main point is, however, that I did not delve into the cosmological problem of the universe's cooling which would require mobilizing concepts of modern cosmology and general relativity, but only addressed Ross's abjectly erroneous, purely thermodynamic explanation of the universe's cooling.

After I shared with Yoachim the above discourse, he sent me the following second message dated May 10, 2004 (reproduced here in full):

"That makes your argument much cleaner--now your beef is clearly that Ross went and used classical thermodynamics on the universe without discussing redshift and scale factors and all that good stuff which really needs to be there for the discussion to be even close to correct. You're welcome to post it if you like, I don't mind.
Keep up the good fight, cheers,

I have nothing to add to this message, and hopefully the above argument will further clarify my critique of Ross' errors for those readers who may still need such a clarification.