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The Cosmological Constant Is Non The Worst Prediction Ever. It’S Non Fifty-Fifty A Prediction.

Think imitation tidings too echo chambers are a work alone inwards political discourse? Think again. You give away many examples of myths too falsehoods on pop scientific discipline pages. Most of them surroundings the hype of the day, only some of them receive got been repeated therefore oftentimes they immediately seem inwards papers, seminar slides, too textbooks. And many scientists, I receive got noticed alongside alarm, genuinely believe them.

I can’t say much almost fields exterior my specialty, only it’s obvious this happens inwards physics. The claim that the bullet cluster rules out modified gravity, for example, is a peculiarly pervasive myth. Another ane is that inflation solves the flatness problem, or that in that place is a flatness work to get with.

I lately constitute some other myth to add together to my list: the assertion that the cosmological constant is “the worst prediction inwards the history of physics.” From that this catchy only incorrect declaration has fifty-fifty made it into textbooks.

Before I larn too brand my case, delight inquire yourself: If the cosmological constant was such a bad prediction, too therefore what theory was ruled out yesteryear it? Nothing comes to mind? That’s because in that place never was such a prediction.

The myth has it that if yous calculate the cosmological constant using the touchstone model of particle physics the number is 120 orders of magnitude larger than what is observed due to contributions from vacuum fluctuation. But this is incorrect on at to the lowest degree v levels:

1. The touchstone model of particle physics doesn’t predict the cosmological constant, never did, too never will.

The cosmological constant is a costless parameter inwards Einstein’s theory of full general relativity. This agency its value must last fixed yesteryear measurement. You tin plow over the axe calculate a contribution to this constant from the touchstone model vacuum fluctuations. But yous cannot stair out this contribution yesteryear itself. So the number of the touchstone model calculation doesn’t thing because it doesn’t stand upwards for to an observable. Regardless of what it is, in that place is ever a value for the parameter inwards full general relativity that volition brand the number fit alongside measurement.

(And if yous yet believe inwards naturalness arguments, buy my book.)

2. The calculation inwards the touchstone model cannot last trusted.

Many theoretical physicists scream back the touchstone model is non a primal theory only must last amended at high energies. If that is so, too therefore whatsoever calculation of the contribution to the cosmological constant using the touchstone model is incorrect anyway. If in that place are farther particles, therefore heavy that nosotros haven’t yet seen them, these volition play a role for the result. And nosotros don’t know if in that place are such particles.

3. It’s idiotic to quote ratios of loose energy densities.

The 120 orders of magnitude refers to a ratio of loose energy densities. But non alone is the cosmological constant ordinarily non quoted equally an loose energy density (but equally a foursquare thereof), inwards no other province of affairs create particle physicists quote loose energy densities. We ordinarily speak almost energies, inwards which illustration the ratio goes downward to xxx orders of magnitude.

4. The 120 orders of magnitude are incorrect to get with.

The actual number from the touchstone model scales alongside the 4th ability of the masses of particles, times an energy-dependent logarithm. At to the lowest degree that’s the best calculation I know of. You give away the number inwards equation (515) inwards this (awesomely thorough) paper. If yous set inwards the numbers, out comes a value that scales alongside the masses of the heaviest known particles (not alongside the Planck mass, equally yous may receive got been told). That’s currently thirteen orders of magnitude larger than the measured value, or 52 orders larger inwards loose energy density.

5. No ane inwards their correct hear ever quantifies the goodness of a prediction yesteryear taking ratios.

There’s a argue physicists ordinarily speak a almost uncertainty, statistical significance, too touchstone deviations. That’s because these are known to last useful to quantify the jibe of a theory alongside data. If you’d bother writing downward the theoretical uncertainties of the calculation for the cosmological constant, the number would last compatible alongside the measured value fifty-fifty if you’d laid the additional contribution from full general relativity to zero.

In summary: No prediction, no problem.

Why does it matter? Because this incorrect narrative has prompted physicists to aim at the incorrect target.

The existent work alongside the cosmological constant is non the average value of the touchstone model contribution only – as Niayesh Afshordi elucidated ameliorate than I ever managed to – that the vacuum fluctuations, well, fluctuate. It’s these fluctuations that yous should worry about. Because these yous cannot larn rid of yesteryear subtracting a constant.

But of course of teaching I know the actual argue yous came hither is that yous desire to know what is “the worst prediction inwards the history of physics” if non the cosmological constant...

I’m non much of a historian, therefore don’t accept my give-and-take for it, only I’d approximate it’s the prediction yous larn for the size of the universe if yous assume the universe was born yesteryear a vacuum fluctuation out of equilibrium.

In this case, yous tin plow over the axe calculate the likelihood for observing a universe similar our own. But the larger too the less noisy the observed universe, the less probable it is to originate from a fluctuation. Hence, the mere fact that yous receive got a fairly ordered retention of the yesteryear too a feel of a reasonably performance reality would last exceedingly tiny inwards such a case. So tiny, I’m non interested plenty to fifty-fifty set inwards the numbers. (Maybe inquire Sean Carroll.)

I for sure want I’d never receive got to regard the cosmological constant myth again. I’m non yet deluded plenty to believe it volition larn away, only at to the lowest degree I immediately receive got this blogpost to scream to when I meet it the adjacent time.
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