Re: Heat death vs zero point energy

This question raises a number of issues for which our understanding is far from complete, as I will try to explain. Firstly, "zero point energy", is a quantum mechanical concept -- it represents the minimum energy a system can have, and unlike classical mechanics, in quantum mechanics this cannot be zero. In fact, this applies not just to systems involving real particles, but even to the vacuum. In quantum mechanics, even completely empty space should have an energy density, which can be thought of as being due to virtual particles (or, if you prefer to think in terms of fields, oscillations of fields representing all possible particles). These virtual particles flicker in and out of existence, but overall should have a dramatic effect creating an extremely high energy density -- much higher than is consistent with observations. For this reason, it was thought that there must be some process which led to the net vacuum energy density being zero.

However, in the late 1990s it was discovered that the expansion of the universal appears to be accelerating. This acceleration can be explained as being due to vacuum energy density, one that produces negative pressure. At first sight, one might think that negative pressure should result in a deceleration, but in General Relativity the effect of this so-called "dark energy" is to create a repulsive force of gravity apparently due to empty space. So, now we have a problem, the universe is behaving as if vacuum energy exists on large scale, but is vastly less strong than it should be. For this reason, physicists are still not confident about the origin of the dark energy, or the details of its behaviour and whether it will evolve in the future.

Now, the second issue is that of the heat death of the universe. This concept arose out of 19th century thermodynamics, and roughly speaking it arises from noting that generally heat in the universe is getting progressively spread from hot to cold, and hence temperature becoming more uniform (and cooler as the universe expands) with time. The reason this is described as "death" is that all life and complex physical phenomena rely on energy transforming and moving, which won't happen if the universe were in perfect thermodynamic equilibrium. [Note, this idea is made more complicated due to the action of gravity, which itself acts to increase entropy - as Hawking deduced in the 1970s - particularly as matter falls into black holes, and black holes merge to create ever larger ones].

Right, so we might now ask, what links these ideas? Well, if the dark energy continues to behave as it seems to in the present era - which is to say not evolving with time - then the acceleration of the universal expansion will itself lead to the universe becoming cooler and less dense at a much faster rate over the coming few tens of billions of years. In that sense, the death will come early. From a practical point of view, though, as a civilisation that relies on energy (and a source of low entropy) in the form of light from a nearby star, the difference is not great - with or without accelerating expansion, the stars will eventually run out of fuel and new stars will cease to be formed.