> This is most likely simply due to the contraction due to the cold ...
> NOT from yeast activity.

Absolutely.

> If you want something sparkling (Champagne, beer, etc), add some
> (carefully measured) fresh fermentables (sugar) at bottling and then
> seal in pressure-proof bottles. That's how homebrew beer is done.
> You'll get a layer of yeast on the bottom of the bottles. There is
> a traditional method that Champagne makers historically have used to
> remove the yeast (riddling) from the bottles, but you can just decant
> off the yeast as well.

It really is pretty much beer making like Derric says here.

I use 10 pressure bottles and sparkle to 6 pressures or 90 PSIG which
is standard sparkling wine pressure.

The key is measuring the sugar added carefully and adding a fresh batch
of different yeast. I might start with a Lalvin D47 at a TA of ~ 8 and
a sugar level of 1.080- 1095; lower than normal starting gravity and
higher than normal acids are normally used. I ferment that dry and
clean it up with bentonite.

Once its done I add 18 grams per bottle cane sugar (as syrup) and the
EC1118 to a carboy and wait until I am sure it's going. (I do 5 gallon
batches). That gets stirred and bottled to around 1 inch from the top
and crown capped.

There are two bottle neck sizes; make sure you get the 'American'
bottles if you already have a beer capper.

That sits out for a few days to make sure the yeast stays active and
then goes to the cellar for a few years. I should turn the bottles
every 6 months but I should do a lot of things...

Riddling is getting the yeast in the neck, easy with a riddling rack.
If you don't want to go that route just keep the boxes and drop with a
twist every day from about an inch and the yeast will slide down.

Disgorging is a bit messy the first few bottles but it's not rocket
science, just technique.

I put 2 bottles in the freezer for 45 minutes and once a plug forms in
the neck i just take it outside, tilt it upright, pop the cap off and
cover with a thumb in one motion. (It's not hard.)Add sugar syrup now
if sweet is desired and cork and wire-hood.

Joe