# Is the earth bombarded equally in all directions by neutrinos?

Perhaps it is hard to known for sure because neutrinos are very difficult to detect, although they do go through the earth in very large numbers. But are they passing through the earth equally from all directions or is it really depending on particular events in the universe. It's probable that the sun generates a lot of them, so perhaps in the daytime we receive more of them than at night. But besides the sun's radiation, are the neutrinos coming from everywhere? Or is it possible that the majority of them comes from one direction, and perhaps even has an influence on the earth's movement?

There are only two types of neutrino source that are "bright" enough to be reliably detected. The sun and nearby supernovae.

The source of solar neutrinos is nuclear fusion, which is also the source of most of the star's energy. Neutrinos also spread out in all directions, so their intensity follows an inverse square law. So the amount of neutrinos is proportional to the brightness of the star. With current detectors no star is bright enough to be observed except for the sun. Other stars do produce neutrinos, and stellar neutrinos come from everywhere (probably more from the milky way) but there are not enough of them to be detected.

Supernovae in the milky way and neighbouring galaxies produce ridiculous amounts of neutrinos, and a spike in neutrinos was observed from SN1987A the closest recent supernova.

Since the sun is the brightest source of neutrinos you might think that the Earth would block out neutrinos during night. However, neutrinos pass right through the Earth almost without noticing. The Earth is transparent to neutrinos. So we detect as many neutrinos at night as during the day.

One thing is certain, there is absolutely no effect on the Earth's rotation or anything else from neutrinos, they just pass right through.

• Addition: Because it is a good question how we can detect neutrinos if they pass through any matter even so thick like planet earth: They sometimes do react with matter. The neutrino detectors are looking for reactions and they only include reactions coming from below. Even the most penetrating cosmic radiation can penetrate earth only some kilometers, so particles coming from under the ground must be neutrinos. – Thorsten S. Apr 6 '16 at 22:18
• To put supernovae into perspective: neutrinos barely react with normal matter at all, as you note. However, if a core-collapse supernova occurred as far from us as the Sun, even the neutrino flux would be enough to kill you. Supernovae are huge. Of course, it wouldn't be the thing that kills you - the light will kill you much faster. – Luaan Apr 6 '16 at 22:24
• @Luaan Please explain. The neutrinos arrive considerably (hours) before the light. – Rob Jeffries Apr 6 '16 at 23:13
• @MasonWheeler, it takes hours for the light from the core-collapse explosion to reach the surface because of all the hydrogen in the way. Neutrinos, on the other hand, barely interact with normal matter, so they reach the surface almost immediately. – Mark Apr 7 '16 at 0:24
• @2012rcampion Roughly speaking it takes a Sievert to kill you and 100 Sieverts to kill you within a few hours. It appears the dose from neutrinos at 1 au in a supernova might just kill you a few weeks later and therefore the comment from Luaan is correct. – Rob Jeffries Apr 7 '16 at 9:09

In addition to neutrinos from the Sun and other discrete sources in the Universe (see James's answer), there is also expected to be a cosmic neutrino background. Although this is yet to be detected (efforts are underway), its expected properties are reasonably well understood. The neutrinos "decoupled" from the universe seconds after the big bang at temperatures $>10^{10}$K. As the universe expands, the de Broglie wavelength of these neutrinos (which are not massless) lengthens with it, such that the neutrinos are expected to have a temperature of $<2$K today. There are 112 of these cosmic neutrinos per cubic centimetre per neutrino flavour (probably 3).

The C$\nu$B is analogous to the cosmic microwave background in a number of ways, but (a) it hasn't been detected; (b) it is cooler; (c) because neutrinos have a small but non-zero mass, the C$\nu$B neutrinos are likely non-relativistic today.

This latter point is important to your question. On large scales we expect the neutrino background to have an asymmetry due to the motion of the Earth through the universe with respect to the co-moving standard of rest. This is exactly the same global dipole asymmetry seen in the cosmic microwave background. However, non-relativistic neutrinos are also anisotropic because they are much more affected by gravitational fields. In particular they should be gravitationally focused by the Sun, such that the Earth receives more neutrino flux when the Earth is "leeward" of the Sun with respect to its motion with respect to the co-moving rest frame. This will produce an annual modulation in any non-directional neutrino flux amplitude of a few tenths of a per cent (Safdi et al. 2014) and might allow a dectection of the C$\nu$B to be confirmed.

On top of this there may be other anisotropies caused by the acceleration of C$\nu$B neutrinos by massive galaxies and clusters of galaxies, that should lead it to be much more inhomogeneous and anisotropic than the cosmic microwave background. Overdensities with respect to the average of factors of 10 or more are possible (see section 2.2 of Yanagisawa 2014), but it depends on exactly what the neutrino mass is.