# Is the Universe electrically neutral at big scales?

Following Prof. Leonard Susskind's Lectures on Cosmology, we stated that the Universe, as we know it, seems to be electrically neutral, from where we catch some consequences of it.

This simple statement makes sense to me but I also wonder: do we have any big proof or experimental results that support it? e.g., if it wouldn't be the case, we would see electromagnetic fields shifting emission lines due to Zeeman/Stark effect.

To our best knowledge, the observable universe is neutral. All evidence at hand suggests that the number of electrons in the universe is (almost) identical to the number of protons.

If tis was not the case, the electromagnetic force, which is $$\sim 10^{39}$$ times stronger than the gravitational force, would dominate in the universe. Earth would not follow Kepler's laws, but instead their motion should be derived from Maxwell equations.

Not having observed such behaviour suggests that gravity is in fact the dominant force on the large scale.

• Thanks a lot for your reply. Could you please extend the "all evidence at hand" fragment, maybe providing bibliography or related links to?
– nuwe
Feb 21 at 15:50
• Universe neutrality is kind of the null hypothesis. As of now, net charge of the universe has not been used to make predictions based on GR or QFT. So the assumption of neutrality seems to make sense to our best knowledge. The second evidence is the one mentioned in the answer: Electric force doesn't dominate over gravitational force (it should if there was a non zero charge). Feb 21 at 16:16
• An interesting answer can be found in "HOW EXACTLY DID THE UNIVERSE BECOME NEUTRAL?" and "Can the Universe Be Charged?". Feb 21 at 16:20
• "Earth would not follow Kepler's laws, but instead their motion should be derived from Maxwell equations." I don't see how. If the Sun and Earth are electrically neutral, how would the charge of the rest of the universe preclude Keplerian motion? Feb 22 at 3:50
• A less immediately absurd model of a charged universe might have intergalactic space filled with a uniformly thin smear of excess electrons. I'd expect radioastronomy can put pretty tight bounds on how many of them there can be, though. Feb 22 at 3:53