In 1918 Einstein informed the gullible world that, during the turning-around acceleration, a HOMOGENEOUS gravitational field appears:

http://sciliterature.50webs.com/Dialog.htm
Albert Einstein 1918: "A homogeneous gravitational field appears, that is directed towards the positive x-axis. Clock U1 is accelerated in the direction of the positive x-axis until it has reached the velocity v, then the gravitational field disappears again. An external force, acting upon U2 in the negative direction of the x-axis prevents U2 from being set in motion by the gravitational field. [...] According to the general theory of relativity, a clock will go faster the higher the gravitational potential of the location where it is located, and during partial process 3 U2 happens to be located at a higher gravitational potential than U1. The calculation shows that this speeding ahead constitutes exactly twice as much as the lagging behind during the partial processes 2 and 4."

This HOMOGENEOUS gravitational field is crucial - without it, the twin paradox becomes an absurdity. The problem is that the HOMOGENEOUS gravitational field itself is an absurdity - it extends from the traveling twin to the stationary twin, no matter the distance between them, and is generated by the turning-around acceleration of the traveling twin, an acceleration which is even absent in some twin paradox scenarios! Actually "absurdity" here is a euphemism - Einstein's 1918 HOMOGENEOUS gravitational field is one of the greatest idiocies in the history of science.

Most Einsteinians implicitly reject Einstein's 1918 idiocy and teach that the turning-around acceleration is immaterial. Sometimes the HOMOGENEOUS gravitational field is mentioned timidly and euphemistically - e.g. here it is referred to as "enough strangeness":

http://www.people.fas.harvard.edu/~djmorin/chap11.pdf
David Morin, Introduction to Classical Mechanics With Problems and Solutions, Chapter 11, p. 14: "Twin A stays on the earth, while twin B flies quickly to a distant star and back. [...] For the entire outward and return parts of the trip, B does observe A's clock running slow, but enough strangeness occurs during the turning-around period to make A end up older."

Pentcho Valev