In addition, I believe that many black holes are expected to be considerably colder than the cosmic microwave background: see http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971111e.html

which states "For black holes the mass of our Sun, the radiation coming from it is so weak and so cool that the temperature is only one ten-millionth of a degree above absolute zero.", and temperature is inversely proportional to mass.

I haven't checked it up, but I would also guess that spontaneous atmospheric or astrophysical masers/lasers could also be lower in temperature.

So unfortunately, I guess we have to answer 'yes' to this one, with the corollary that just finding cold things won't help us with alien life.

Since its launch on May 14 (accompanied by its sibling spacecraft Herschel), Planck has been traveling to its final orbit at the second Lagrange point of the sun-Earth system, L2, and cooling its instruments down to their operational temperature of minus 459.49 degrees Fahrenheit (minus 273.05 Celsius). This temperature is just 0.1 Celsius above absolute zero,

@Louis. What would be the physical explanation for being colder than the background?

Derek,

The temperature of the CMB depends on the temperature of the universe when the CMB radiation was emitted and the lowering of this radiation by the expansion of the universe during the last 13.6 billion years. I do not understand why you think that this temperature should be a ground temperature?

One may consider the question for a question about the measure of energy. The brightness temperature is the measure of radiation energy observed. Therefore, it is possible to observe less than 2.72K if only observational means allow. Total energy observed can be represented by the Stokes vector, with the first vector element assigned to the intensity which must be real and positive. Otherwise one should discuss the observational means. This is my point of view, that assigning the brightness temperature to the temperature of an object is quite another question. The Boomerang Nebula is beautiful and seems to be real.

The CMBR temp. of about 2.72 degree-Kelvin is the average temperature. But due to inhomogeneities of baryonic matter at the last scattering surface, the CMBR temperature varies with direction - cooler in some directions and hotter in others. Although the variation is only of the order of few times 10^{-5} degree-Kelvin. And it is long lasting.

The temperature is a measure of one of:

kinetic energy with respect to some frame,

or of the ratio of population of two states,

or a distribution and density of energy in the radiation field..

In general the universe is not in thermal equilibrium, and these temperatures are not the same.

All masers and lasers work with an inverted population of states so that their

corresponding temperature is negative. Thus the many natural masers all correspond to temperatures lower than the microwave background temperature.

The cosmic Background Radiation is in a very good approximation a black Body Radiation. Every Body which is in a thermal Equilibrium with this Radiation source has this temperature. If you have another Radiation sources, usually hotter bodies like suns, the temperature increases.

If due to the surface reflectivity the Absorption is low, the Body temperature approximates to a lower value, that Emission and Absorption are equal. Therefore it might be possible, that bodies consisting of ice or snow and having a clean surface, have a temperature below cosmic Background temperature. This occurs only far away from any other Radiation source.

I would look for comets out of our sun system

The coldest thing in the Universe was the HFI instrument on-board Planck satellite, cooled down at 0.1K.

Unlikely, apart from laboratory conditions. Since the Big Bang is fantacy, a violation of known physical laws. An infinite, steady state Universe, free from known law violations on a macroscopic scale, is far more likely and must produce some CBR. Some lumpiness due to galaxy clusters would be expected and is found in the measurements. Doppler effect red shift has more prosaic explanations (eg. energy loss by photons) and quasars travelling towards the Milky Way are highly red- shifted and comparatively close-no Doppler effect! They appear to be new galaxies in formation, ejected from the parent galaxy. The CBR is also lumpy, not in accordance with BBT ( Hoyle's term of derision).

Regards,

David Raal.

"An infinite, steady state Universe, free from known law violations on a macroscopic scale, is far more likely"

Indulge me, how do you deal with ever increasing metallicity or, say, the second law of thermodynamics.

Dear Suresh Kumar, Can you provide me that another related thread you have referred to? I would like to watch and read this discussion. Wojciech Marczewski

Negative temperatures are a well know phenenomenon in atomic physics. If you invert the populations of the electrons in an atomic system the temperature is by definition negative. thermodynamically such systems will always tend to lose energy to their surroundings, which means that they are in fact hotter than any object or system with a positive temperature, so definitely not the coolest systems in the universe

Very nice argument by Porfessor John Beckman. I am agreed with his opinion.

The second law of thermodynamics says that we can not take away the heat from a point or place by cooling it below the coldest of the surrounding. Hence, there can not be place in the universe where the temperature is lower than the cosmic back ground.

However, in dark mater, in black hole etc, only the experts in these area should give answer.

Suresh´s answer reveals a difference between mathematics and physics. It is true that in mathematics negative numbers are, by definition, lower than positive numbers. However the definition of a negative temperature, accepted fully and used by the physics community, implies that negative temperatures are (in the sense of the second law of thermodynamics) higher than positive temperatures. This appears paradoxical, but it is a matter only of the definitions in both cases.

Oh, excuse me exaggerating - Is the mathematics less real than physics due to some duality notified? No, math allows to see new aspects which were not present under other considerations. In my understanding the discussion touches or avoids some singularities. Since few days I think that usually there are some distinct singularities in particular concepts which may occur caused by a common reason. It seems possible that reasons may be in taking the time domain span from negative to positive values, but not only the time. One may want considering the state of equilibrium and/or inflation and displays arguments. Others may focus on available physical act of observation and ask not for that what it is, but seek an answer on that what is the value observed. Then two different persons may discuss different subjects and successfully state with own arguments. Do you know real observations of cmb in negative real values for the temperature brightness? It is technically and physically possible but why remaining only with the intensity of TB?

Wojciech . Please read my previous comment about negative temperatures in physics:how they are defined, and how they are used. There is no contradiction between this and the definition of negative numbers in mathematics. They are just

different concepts in different disciplines.

Even the absolute zero is impossible according to the basic law in Physics, then what about the negative temperature? Mathematical concept may or may not be closer to the reality.

A few comments: as to negative temperatures: if one allows two objects at two positive temperatures to come into thermal contact, heat will flow from the higher temperature to the lower. This is the factual basis why we say that higher temperatures are hotter than low ones, not the simple fact that one of the temperatures is a larger number than the other. When an object with a negative temperature is similarly put in thermal contact with an object having positive temperature, heat flows from the object having negative temperature to the one that has positive temperature. In that sense, objects with negative temperature are hotter than those having positive temperature, without, of course, disputing the basic fact that negative numbers are less than positive ones. Perhaps it helps to say that the inverse temperature might be the more natural parameter to use.

Indeed, absolute zero is unattainable, but for spin systems weakly coupled to the rest of the world, negative temperatures are in fact a useful concept. Indeed, to reach a negative temperature, one does not generally pass through zero, but rather, in some sense, through infinity.

As to the second law: it does not, in fact, prohibit cooling below the lowest temperature in the surroundings: otherwise refrigerators would not be possible. What it requires is that there be a *source of energy* to pump the energy away from the refrigerator into the surroundings. Whether this can happen in the Universe in a situation where no intelligent life is present is not altogether obvious to me. However, adiabatic expansion of a gas does indeed cool it substantially---it is in fact the operating principle for ordinary refrigerators on Earth---so expanding gas could certainly be cooler for a rather long time than the Cosmic Background Radiation.

This is an excellent answer, and incorporates some of the rather less explicit remarks which I made before, adding some clear and useful points. Thanks

Of course it is possible to be colder than the surroundings! We have dilution refrigerators that continuously keep temperatures around 5-10mKelvin. ADR systems can keep temperatures below 1 mK for months. Below that it gets tricky about describing temperature since the nuclei and electrons in materials will have different temperatures. Nuclear temperatures in the nanokelvin and high pico kelvin range are possible. Way below the cosmic background. The coldest places in the universe are on earth.

Negative temperatures are for systems not in equilibrium. A laser has negative temperature since there is a higher population in the higher energy level. The way to describe this is with a negative temperature but it is a metastable state that will release energy as the temperature becomes positive and the population drops to the lower energy level (i.e. laser radiation).

Negative temperature is not just a mathematical trick. It is a physical way to describe the occupation of energy levels in a system. But it is just non-physical to claim that it has a gravitational effect.

This paper deals with the problem under consideration:

NATURE | NEWS

Quantum gas goes below absolute zero

Ultracold atoms pave way for negative-Kelvin materials.

Zeeya Merali

03 January 2013

Nature doi:10.1038/nature.2013.12146

@Uryson,

The Nature paper is the creation of a population inversion. In this case they changed the magnetic field such that all the atoms were in the upper energy state. For example if you have a system where there are two energy levels for each atom, then as you heat it up all the atoms will end up equally in each of the two energy levels with a population probability of 50%. At high temperatures the atoms have so much energy that the difference between the energy states is insignificant and the probability goes to 50%. If you cool it down as it reaches zero temperature you end up with all the atoms in the lowest energy state.

The only way to get a population that is mostly in the higher energy state is to use the concept of negative temperatures. This is called a population inversion.

The mathematics is that the atoms (or any particles) obey the Maxwell-Boltzman statistics. http://en.wikipedia.org/wiki/Maxwell–Boltzmann_statistics

The contributions by S. Hannahs are clearly entirely relevant, and add a lot to what has gone before. When I made my contribution to this discussion my plan was not to go into physical detail but just to give some guidelines. Subsequent contributors have amplified and of course thereby greatly improved what I said, with specific examples and more details. I must say that the Nature headline is sensationalist when it says "Quantum gas goes below absolute zero". It leads readers to believe that in some way negative temperatures are lower than absolute zero. This is a contentious way of describing the requirement to use a negative index in the Boltzmann expression when you have an inverted population. However as Nature is often a "pop" journal this should not surprise people.

Dear Professor S. Hannahs

Very nice comment, Really, I am agreed.

But as I know that particles also follow the "Bose Einstein statistics" along with the Maxwell-Boltzman statistics. And it depends on the charge of particles Either it is integer charge or fractional charge.

Dear Professor S. Hannahs

your comment is very clear.

Of course it is possible to find temperature lower than the cosmic background (CB). It is not possible only if an object reaches a thermal equilibrium condition with the CB, but a disequilibrium system (an expanding gas system is the most obvious example) could in principle cool at any temperature for a long time before being heated by the CB.

Cosmic background radiation is microwave radiation. If one can shield it, one can have temperatures lower than microwave background radiation in shielded region for a short time. Essentially one is isolating the system from rest of the world. In laboratories we do have temperatures lower than 3^o K. Superfluidity occurs in He^4 at about 2 degrees kelvin and superfluidity in ultracold atoms occurs at nanokelvins and is seen in laboratories.

The original question asked whether an observation of a temperature below the microwave background would be an indication of intelligent life. The answer is definitely NO. If an intelligent species wished to create and maintain a temperature below the microwave background, one of the first decisions would be to uncouple it thermally from the rest of the universe, so as not to waste energy in continually cooling it to compensate for the heat flow inwards.

Since the cool region would have been thermally uncoupled from the rest of the

universe, an outside observer would not know that the super-low temperature had

been attained.

Therefore, the only super-low temperatures that would be observed would NOT have been produced by an intelligent organism. Likewise, observing the products of radioactive decay in the atmosphere of a planet would not be an indication of the

presence of intelligence. It might however be an indication of the presence of an

organism that was clever but stupid. And in the same way, one cannot rule out that

observation of a super low temperature was the result of activity of an organism that was clever but stupid.

dear Woolf,

why we couldn't see super-cooled regions from outside if thermally uncoupled from the cosmic background? Thermalization with the cosmic background could be avoided (or delayed) filtering out the microwave spectral region, but emissions at longer wavelengths may in principle allow measuring the lower temperatures inside the system from outside.

The cosmic microwave background is isotropic blackbody radiation that fills the entire universe. Its present temperature is about 3 K. Its existence is evidence that the universe expanded from a hot, dense state. As the universe has expanded, the initially high-energy radiation has been red-shifted to lower and lower temperatures. At the present time, the density of matter in the universe greatly exceeds the equivalent mass density of radiation. We live in a matter-dominated universe. The matter density was much greater in the past, when the universe was smaller. However, because radiation is red-shifted as the universe expands, the density of radiation was greater still. The early universe was radiation dominated.

Most of the helium observed in the universe today is primordial, created by fusion between protons and neutrons in the early universe; this is known as primordial nucleosynthesis. Detailed studies of this process indicate that "normal" matter can account for at most three percent of the critical density. By the time the universe was about 1500 times smaller than it is today, the temperature had become low enough for atoms to form. At that time, the (then visible) radiation background decoupled from the matter. The photons that now make up the microwave background have been traveling freely through space ever since.

much more description has been depicted in the following web link:-

http://www.castlerock.wednet.edu/HS/stello/Astronomy/TEXT/CHAISSON/BG317/HTML/BG317EOC.htm