Effects of a different CMBR temperature?

[vicky_molokh]

Originally Posted by Wavefunction

If it's a small different then there likely wouldn't be much of a change. The universe has been consistently expanding for billions of years, this expansion leads to the gradual reduction of the CMB temperature. This is because the energy density of radiation is inversely proportional to the scale of the universe raised to the fourth power, thanks to the effects of volume and wavelength - increasing wavelength decreases frequency, whic decreases the energy of radiation.

[ . . . ]

In short, a higher CMB temperature wouldn't produce much of an effect, unless you're talking about a rather more extreme increase than I think. In which case, I'd need some time to think it over.

What do you count as a little, and what is (the beginning of) extreme, and what would be the sensor modifier effects of the former and the latter?

[rknop]

Use Wein's Law to figure out where the peak wavelength is. If it's not very close to or within a band where you're detecting things, then it probably won't matter much.

The intensity is going to scale as T^4, which is a lot -- but, even still, it's not very bright right now. Remember that to detect it, we have to use balloon-borne or space-based experiments, and even then there's a lot of other background and such to subtract.

What kinds of things are you trying to detect? With an example, I could probably give a back-of-the-napkin estimate of how much hotter the CMB would need to be for it to make a difference.

Also, it wouldn't change how we calculate blackbodies. The CMB may be our most perfect example of a blackbody, but the constants used (k and h) are measured in other places. I have a vague memory of the ultimate calibration of blackbodies coming from an experiment done at some observatory with molten platinum....

[whswhs]

Originally Posted by rknop

Also, it wouldn't change how we calculate blackbodies. The CMB may be our most perfect example of a blackbody, but the constants used (k and h) are measured in other places. I have a vague memory of the ultimate calibration of blackbodies coming from an experiment done at some observatory with molten platinum....

It wasn't molten platinum, if I remember my freshman physics correctly. It was a hollow cavity inside a solid block of platinum. You need it to be a hollow cavity to make the effects of albedo irrelevant. Checking Halliday and Resnick, I see they say it also makes the size and shape of the cavity irrelevant.

[whswhs]

So, okay, on one hand, mean energy per unit volume varies as L^(-4). On the other hand, minimum mass for gravitational collapse to form a solar system varies as T^(3/2). To link the two, we need a conversion between energy and temperature.

* In basic thermo, temperature was defined as mean molecular kinetic energy, which would suggest that T is proportional to E, probably using Boltzmann's constant or the ideal gas constant.

* In blackbody radiation, energy radiated per unit time is proportional to T^4.

* However, the universe doesn't seem to be radiating into anything other than itself. I believe the proportionality for energy *content* in a medium is that internal radiation is proportional to T^3.

Which of these gives the right scaling relationship?

[Anders]

The first stars formed when the universe was about 200 million years after the Big Bang. How hot would the CMB be then.

[malloyd]

Originally Posted by vicky_molokh

What do you count as a little, and what is (the beginning of) extreme, and what would be the sensor modifier effects of the former and the latter?

Generally I'd put it at at least half the effective temperature of the thing you are trying to detect to have any effect at all - at that point it's still radiating only 1/16th as much per unit area (and mostly not even at the same frequencies). A sensor so lousy it can't pick out something 16 times brighter than the background with no frequency discrimination at all is worthless junk.

That's about where it matters for blackbody temperatures too, adding back 1/16th the energy you are radiating away raises your (kelvin) temperature 1.53%

[Fred Brackin]

So if Malloyd thinks you've got to get up half of a human-crewed spaceships 270 K to make a difference that far beyond the boiling point of hydrogen and I'm not sure even gas giants exist. Icey bodies almost certainly won't.

As to the cosmological implications I think the universe has to be a lot smaller and younger. The initial temp of the background radiation is fixed by the laws of physics. You don't get the "Big Bang" flash until space has cleared out enough for the photons to fly between the electrons and protons without being absorbed. That sets things to happen at a given density so making the universe have more mass wouldn't change it.

So there probably hasn't been enough time for third generation stars to appear and planets to cool and life to evolve.

So unless the universe's physics are radically different I don't think any beings similar to ourselves would ever face such a problem.

[malloyd]

Originally Posted by Fred Brackin

So there probably hasn't been enough time for third generation stars to appear and planets to cool and life to evolve.

Doing the most simplistic calculation - age of universe/(ln(270/2)/ln(2.7)) - you'd expect that temperature about 470 million years after the beginning. That is slightly after stars have started to form, but not by very much. Which makes sense, stars presumably won't form until it's cooler than their surface temperatures, and on an exponential curve stars are not actually a lot hotter than 270K.

[vicky_molokh]

Originally Posted by malloyd

Generally I'd put it at at least half the effective temperature of the thing you are trying to detect to have any effect at all - at that point it's still radiating only 1/16th as much per unit area (and mostly not even at the same frequencies). A sensor so lousy it can't pick out something 16 times brighter than the background with no frequency discrimination at all is worthless junk.

That's about where it matters for blackbody temperatures too, adding back 1/16th the energy you are radiating away raises your (kelvin) temperature 1.53%

IOW, even if you raise it to 30K or even 100K, this will have zero noticeable effect on planetary climates?

Originally Posted by Anthony

Unless you're scanning at microwave wavelengths, changes in the CMB will not directly affect detection at all. However, differences in the expansion of the universe probably mean there's also changes in the backgrounds of galaxies, which will have some effect.

Will it have any effect on the 'space is very cold' factor of detection under any circumstances (such as ways of radiating away waste energy in an adjusted spectrum, perhaps with some ultra-tech advancements)? Or none either?

[malloyd]

Originally Posted by vicky_molokh

IOW, even if you raise it to 30K or even 100K, this will have zero noticeable effect on planetary climates?

Well for terrestrial planets. It may matter for iceballs. It's much the same issue as the second stars of binary systems usually don't matter for the habitable planet's temperature.

Two temperature sources add as the fourth root of the sum of their fourth power. So if you have two sources that would heat something to 100K in isolation, their combination heats the thing to 119K, which could matter, but if you have a one source that heats it to 300K and add another that would heat it to 100K, the combination heats it to 300.9K, which is likely negligible.

Edit: Is there a goal here? If you want a setting where space happens to be warm, I wouldn't suggest tampering with cosmology. Look for an excuse to bathe the entire region of the setting in a hot gas cloud instead. Yeah it's a little tricky to justify why whatever heated a cloud light-years across didn't kill everything in the region, but you can probably come up with something. If another galaxy collided with the Milky Way you might be able to get a jet of gas getting tossed off in the direction of one of the Magellanic Clouds that would still be fairly warm when it got there with less handwaving than changing the expansion of the universe. Sure it'll pass through, or cool back down again in 10 or 100 million years, but your metaplot doesn't need that much time anyway, right?