does antigravity exist?
is there a justifiable physics behind it?
No. Absolutely none.
Maybe, just maybe, "not yet"??
Astrophysics is the understanding of the universe 'out there' (astro-) by the application of the 'physics down here' (-physics).
Sometimes, however, just occasionally, it works the other way round and we learn about something 'out-there' that is subsequently discovered in the laboratory 'down-here'. So, for example, helium was discovered in the solar spectrum before it was identified in the laboratory on Earth.
Today in cosmology the 'physics down here' is the application of GR and nuclear physics to the universe on the largest scales and earliest times.
Interpreting the data under that paradigm it is found that physics, as yet undiscovered in the laboratory, has also to be invoked to produce a concordant standard model; viz: Inflation with its Higgs Boson or Inflaton, non-baryonic DM and DE.
The last one is the interesting one from this question's POV, because on a cosmological scale it performs the role of anti-gravity.
Does DE only work on the largest scales, say as the cosmological constant, or is it another kind of repulsive force?
If the latter, will DE ever be identified in the laboratory?
Does it actually exist in the first place or is the need for these entities an artifact of the standard paradigm breaking down at these largest ranges and earliest times?
I would advocate keeping an open mind at the moment until we know more about what we are talking about.
Negative energy has been well-supported by theoretical model and somewhat verified by experiment. It is predicted to be "gravitationally repulsive", but experimentation to prove that would be extremely difficult. Heck, as far as I know, we haven't even verified the gravitational behavior of antimatter yet. It's predicted to behave "normally" in response to the presence of a gravitational field, but gravity's just so weak that it's hard to measure.
So I would say yes, there is "justifiable physics" behind it, but getting emperical proof is nearly impossible.
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