Black Screen when booting Ububuntu 12 on iMac

Black Screen when booting Ububuntu 12 on iMac

here is a summary of what i done so far:
install rEFT and create a bootable usb device with unetbootin (ubuntu 12.04) on osx
reboot and install ubuntu 12.04 with no errors
boot GRUB from the rEFIT
boot Ubuntu from GRUB
Ubuntu logo pops up and then i got a black screen
Doesn't change when i hit str-alt-F(1-10)
I have found a lot of log-files, maybe someone can understand it?
I think it maybe the NVIDIA ? or something wrong with the x-org?
dont even know what that is, xorg ...



here are my specs:
imac 2012, 2.7 ghz intel core i5, NVIDIA geforce gt 640m, 8 gig ram, 1 tb hdd



and here is the boot.log on the ubuntu partition:
fsck von util-linux 2.20.1 fsck von util-linux 2.20.1 dosfsck 3.0.12, 29 Oct 2011, FAT32, LFN /dev/sda4: sauber, 180336/6914048 Dateien, 1305308/27653888 Blöcke /dev/sda1: 17 files, 47597/403218 clusters
modem-manager[855]: ModemManager (version 0.5.2.0) starting...
modem-manager[855]: Loaded plugin Wavecom
...
modem-manager[855]: Loaded plugin Sierra
Skipping profile in /etc/apparmor.d/disable: usr.bin.firefox
Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
Starting AppArmor profiles [240G [234G[ OK ]
speech-dispatcher disabled; edit /etc/default/speech-dispatcher
saned disabled; edit /etc/default/saned
Stopping System V initialisation compatibility [234G[ OK ]
Starting System V runlevel compatibility [234G[ OK ]
Starting restore sound card(s') mixer state(s) [234G[ OK ]
Starting LightDM Display Manager [234G[ OK ]
Starting ACPI daemon [234G[ OK ]
Starting anac(h)ronistic cron [234G[ OK ]
Starting save kernel messages [234G[ OK ]
Starting crash report submission daemon [234G[ OK ]
Starting CPU interrupts balancing daemon [234G[ OK ]
Stopping restore software rfkill state [234G[ OK ]



and i found this Xorg.0.log :
...
[ 15.620] (II) NVIDIA: Using 3072.00 MB of virtual memory for indirect memory
[ 15.620] (II) NVIDIA: access.
[ 15.624] (WW) NVIDIA(0): ACPI: AC power state information is not available under
[ 15.624] (WW) NVIDIA(0): /sys/class/power_supply/ , nor under
[ 15.624] (WW) NVIDIA(0): /proc/acpi/ac_adapter/
[ 19.625] (EE) NVIDIA(GPU-0): EVO Push buffer channel allocation failed
[ 19.625] (EE) * Aborting *
[ 19.625] (EE) NVIDIA(GPU-0): Failed to allocate EVO core DMA push buffer
[ 19.625] (EE) * Aborting *
[ 19.625] Fatal server error:
[ 19.625] AddScreen/ScreenInit failed for driver 0
[ 19.625]
[ 19.625] (EE)
Please consult the The X.Org Foundation support at http://wiki.x.org for help.
[ 19.625] (EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[ 19.625] (EE)
[ 19.629] Server terminated with error (1). Closing log file.



and also the failsave-x.log:
...
Initializing built-in extension XFree86-DRI
Initializing built-in extension DRI2
Loading extension GLX
vesa: Ignoring device with a bound kernel driver
vesa: Ignoring device with a bound kernel driver
Fatal server error:
no screens found
(EE)
Please consult the The X.Org Foundation support at http://wiki.x.org for help.
(EE) Please also check the log file at "/var/log/Xorg.failsafe.log" for additional information.
(EE)
Server terminated with error (1). Closing log file.
xinit: giving up
xinit: unable to