Fill in the blanks:

1. the is the boundary of a black hole beyond which a spacecraft could not even in principle transmit a radio signal to observers outside the black hole. 2. a star that happens to be hidden behind the sun might become viewable during a solar eclipse due to the bending of light known as . 3. according to relativity, at the center of a(n) spacetime becomes infinitely curved. 4. two neutron stars orbiting each other at close distance will gradually spiral in toward each other because they lose orbital energy to . 5. if a(n) can really exist, it might be able to connect two widely separated locations in the universe. 6. is the slowing of time that you observe when you look at an object with a strong gravitational field. 7. spectral lines of hydrogen on a white dwarf appear at longer wavelengths than the same lines observed in the spectrum of the sun, illustrating what we call .

The event horizon is the surface of a black hole, it is the border of space-time in which the events on one side of it can not affect an observer on the other side.

That is, at this border also called "point of no return", nothing can escape (not even light) and no event that occurs within it can be seen from outside.  

According to Einstein's theory of relativity, mass causes the curvature of spacetime and this curvature is what we call gravity.

Then, the light always move following the curvature that gravity causes in spacetime, and in this way the rays of light are deflected if there is a mass accumulation nearby. Being this is the principle of the gravitational lensing.

According to Einstein's theory of relativity, inside a black hole there is a "singularity" that consists of a region of the space in which the density of matter tends to infinity.

How is it possible?

Following what relativity establishes, bodies within a gravitational field follow a curved space path. Then the more a body enters the black hole, the more curved the space will become, until, in the center, it will become infinitely curved.

Gravitational waves were discovered (theoretically) by Albert Einstein in 1916 and "observed" for the first time in direct form in 2015.

These gravitational waves  are fluctuations or disturbances of space-time produced by a massive accelerated body, modifying the distances and the dimensions of objects in an imperceptible way.

In this context, an excellent example is the system of two neutron stars that orbit high speeds, producing a deformation that propagates like a wave, in the same way as when a stone is thrown into the water.

A wormhole is a hypothetical feature of a space-time described in the equations of general relativity, which essentially consists of a shortcut through space and time.  It is not proven that they exist, although mathematically they are possible .

In other words:

A wormhole is a tunnel that joins two black holes in different places (two points of spacetime).

The dilation of time is a phenomenon that results from the theory of relativity, which states that time passes at different rates in regions of different gravitational potential.

That is, the greater the local distortion of spacetime due to gravity, the slower the time passes.

When we talk about the visible electromagnetic spectrum, we know it starts in violet-blue and ends in red.  

Now, in this context the astronomer Edwin Powell Hubble observed several celestial bodies, and when obtaining the spectra of distant galaxies he observed that the spectral lines were displaced towards the red one (red shift), whereas the nearby stars showed a spectrum displaced to the blue one.  

From there, it was deduced that the farther an body in the universe is, the more redshifted it is in its spectrum.  This effect is due to the dilation and contraction of time by speed.  

measuring things 1 litre of water