Definition of temperature:
1/T = dS/dE T is temperature S is Energy E is entropy.
meaning temperature depends on how entropy (S) changes when you add energy (E).
Key idea
In normal systems, adding energy increases disorder (entropy), so temperature is positive.
In some special quantum systems with a maximum possible energy, adding energy beyond a point actually reduces entropy.
This flips the sign of , making temperature negative.
What “negative temperature” really means
It is not colder than 0 K.
It is actually hotter than any positive temperature, even hotter than infinite temperature.
If a negative-T system touches a positive-T system, heat flows from negative to positive temperature, because the negative-T system is in a highly energetic, inverted state.
Conditions required
A system can only have negative temperature if:
It has a finite upper energy bound (not all systems do).
Examples include:
Quantum spins
Atoms in optical lattices (band-limited systems created with lasers)
Classical gases (like air) cannot reach negative temperature because their energy is unbounded above.
Experimental realization
Atoms are trapped in an optical lattice (laser-created periodic potential) like for electrons in semiconductors. Band formation of quantisation of energy states is a result of confinement of electron in a region or space.
This creates a band structure with both lower and upper energy limits.
Atoms are engineered into the highest-energy states while still remaining in equilibrium → giving a stable negative temperature state.
Physical interpretation
Positive temperature: particles prefer low-energy states.
Infinite temperature: all energy states equally populated.
Negative temperature: high-energy states become more populated than low-energy ones (population inversion).
Important clarifications
Negative temperature systems are not unstable explosions or extremely energetic in absolute terms; they often contain less total energy than everyday hot matter.
They are “hot” only in the thermodynamic sense (energy flow direction), not in everyday heat content.
Analogy with Gravity negative Pressure and Dark energy
Negative absolute temperature is a well-defined thermodynamic state for special quantum systems with an upper energy limit, where adding energy reduces entropy, making the system thermodynamically “hotter than infinite temperature.”
The space is seen to be in very similar state as well in its own. The space is in acceleration and expanding when it should slow down. It is like a ball rolling up an infinitely high hill.
Cosmologist call it Dark energy.
It is due to dark matter. It is matter that doesn’t interact with light. Then I say must not be seperate from light. More on that later.
Pressure is defined as:
P = -dE/dV E is energy V is Volume.
Minus sign tells for positive pressure E and V must have inverse relation.
In microscopic scale like LPG Gas in a cylinder it has been found always positive.
Negative Absolute Pressure is not possible. Only relative negative pressure is possible.
But in Space they say Gravity effect decreases with distance.
Imagine
Throwing a ball upward. It goes up, but gravity slows it down.
Even if gravity becomes weaker, the ball still slows down—it just slows down less.
Why doesn’t it start speeding up?
Because:
gravity is still a “pulling back” force
even if weak, it still acts like a brake
So:
> weak brake ≠ accelerator
it just means “less braking”
So what is dark energy?
Normal matter (stars, planets): slows expansion (like gravity brake)
Weaker matter density: weaker brake, but still braking.
Dark energy: not a brake → it’s like a built-in acceleration.
> Less gravity only removes slowing down.
Dark energy is what actually pushes expansion to speed up
It’s like negative Absolute pressure.
P = -dE/dV E is energy V is Volume.
E and V have direct relation.
Now I don’t know all the Friedmann acceleration equation or General relativity equations much. But if entropy can decrease with energy in a manual setup in the lab such a situation for gravity in space must already be in place.
I am just intuitively saying that dark energy is Energy from entities beyond the boundaries of our universe.
But in our model wether it has an end or not we don’t know.
Our body have an end or boundary which infact is illusion but for studying physics nothing more is considered as us.
To study space with equations universe also has an end. Beyond that is the reflection of this universe in metaphorical sense. Not in same physical sense. Scientist call it Dark matter.
For example metaphorically we say I and the tree both are the same. One inhale oxygen the other exhale and similarly for Carbon dioxide.
This if I want to entertain my Ego I can stretch to every thing other than my body.
Similarly I can more entertain my Ego by saying my universe and universe beyond mine. Scientist call it Dark matter.
And it must be a loop to minimize getting lost and turn entertainment into overwhelmingness.
Only this is concluded from observable universe that dark matter is much more than normal but it is a formless matter. Normal matter just pass right through it.
So this universe is result of Big Bang. The outside universe is a result of Big collapse. The centre or the original location the collapsing universe beyond.of big bang in also the centre of collapsing universe.
It is hard to imagine because of the fourth dimension.
The expanding universe is a collapsing universe in fourth dimension.
The collapsing one is expanding in three.
Where or when does it end ?
This is again overwhelming and so unnecessary because we will never live that long. So we can say it just change roles when and where is trying to ask more than our or bain of anybody in this universe brain can know.