It seems to me that to have a basis for ethics one has to believe that some things are valuable. If nothing has any value then no action can have any motive: for acts always seek to obtain that which is good for or valuable to the agent - else they are irrational acts.
Now, if the entire universe is futile and will "come to nothing" as Physics predicts; then it follows that everything that presently exists and will exist and has been achieved and will be achieved is futile. Hence there is no point in anything and no possible purpose to life. This is the conclusion of the Nihilists, and in the terms which they set themselves I am sure that they are correct.
Faced with this harsh reality, one has three options:
1. Adopt Nihilism - but this is incoherent, for to do so indicates that there is a motivation for an act (the act of becoming a Nihilist) but this is opposed to Nihilism.
2. Adopt Existentialism - claim that somehow human reality generates value "ex nihil". However this is incoherent as it is impossible for something which is intrinsically valueless (that is "human reality") to generate value. All that it can generate is relative value: "value to human life" - but if "human life" is of no value itself, this relative value is worthless, being relative to nothing-of-value.
3. Adopt Eternalism - claim that there is an aspect of reality (yet to be determined) which is not futile and is of an eternal and unchanging and imperishable nature. Once one adopts this position, value and worth spread outwards from this supposed eternal dimension of reality in a manner which it is possible to account of rationally.